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The influence of periodic wind turbine noise on infrasound array measurements
Abstract
Aerodynamic noise emissions from the continuously growing number of wind turbines in Germany are creating increasing problems for infrasound recording systems. These systems are equipped with highly sensitive micro pressure sensors accurately measuring acoustic signals in a frequency range inaudible to the human ear. Ten years of data (2006–2015) from the infrasound array IGADE in Northern Germany are analysed to quantify the influence of wind turbine noise on infrasound recordings. Furthermore, a theoretical model is derived and validated by a field experiment with mobile micro-barometer stations. Fieldwork was carried out 2004 to measure the infrasonic pressure level of a single horizontal-axis wind turbine and to extrapolate the sound effect for a larger number of nearby wind turbines. The model estimates the generated sound pressure level of wind turbines and thus enables for specifying the minimum allowable distance between wind turbines and infrasound stations for undisturbed recording.
... In recent years, the debate in Germany rose and had controversy on the level of such infrasound emitted by wind turbines. In our view, this debate is mainly rooted in a misinterpretation of the infrasound signal of a wind turbine measured by Pilger and Ceranna [1]. ...
... Pilger and Ceranna [1] measured the outdoor pressure signal nearby a wind turbine and presented a spectrum with a short excerpt of the time series in their work. The amplitudes of their spectrum are the basis for their discussion and have been referenced by others. ...
... The levels of the harmonics are higher than the estimated sound pressure level. Pilger and Ceranna [1] do not supply sufficient details in their work to identify the scaling of the presented spectrum unambiguously. The inconsistency and the incomplete description of the data analysis have motivated us to reanalyze the publicly available time-series data 1 [8]. ...
The infrasound levels due to the blade-tower interaction generated by a wind turbine in the publication by Pilger and Ceranna (JSV, Vol. 388, pp. 188–200, 2017) have to be corrected to be interpreted as sound pressure level. Also, the electrical power of the wind turbine should be corrected for the high wind case to 660 kW. We provide a reanalysis of the measured data with a power spectrum showing levels for the low-frequency signal of the wind turbine about 34 dB below the original work. All measured levels at a distance of 200 m from the wind turbine's infrasound signal are well below the hearing threshold.
... In recent years, the debate in Germany rose and had controversy on the level of such infrasound emitted by wind turbines. In our view, this debate is mainly rooted in a misinterpretation of the infrasound signal of a wind turbine measured by Pilger and Ceranna [1]. ...
... Pilger and Ceranna [1] measured the pressure signal nearby a wind turbine and presented a spectrum with a short excerpt of the time series in their work. The amplitudes of their spectrum are the basis for their discussion and have been referenced by others. ...
... The levels of the harmonics are higher than the estimated sound pressure level. Pilger and Ceranna [1] do not supply sufficient details in their work to identify the scaling of the presented spectrum unambiguously. The inconsistency and the incomplete description of the data analysis have motivated us to reanalyze the publicly available time-series data 1 [8]. ...
The infrasound levels due to the blade-tower interaction generated by a wind turbine in the publication by Pilger and Ceranna (JSV, Vol. 388, pp. 188-200, 2017) have to be corrected to be interpreted as sound pressure level. Also, the electrical power of the wind turbine should be corrected for the high wind case to 660 kW. We provide a reanalysis of the measured data with a power spectrum showing levels for the low-frequency signal of the wind turbine 34 dB below the original work. All measured levels at a distance of 200 m from the wind turbine's infrasound signal are well below the hearing threshold.
... This is an important noise source, as it can be of major importance for various rotating systems ranging from helicopter rotors to modern upwind wind turbines. For the case of upwind wind turbines, it has been recently demonstrated that BTI noise can have an effect (mainly via providing increased background noise) on measurement systems used for detecting seismic activity and nuclear explosions [1,2]. BTI also creates impulsive torque changes, which may contribute to a reduction in the fatigue life of the gearbox [3]. ...
... For both separation distances, the contribution from the tower is dominant and increases with frequency. Such a spectral shape in the infrasonic frequency region is commonly observed in the vicinity of modern industrial wind turbines [1,31] with its hypothesized origin to be the blade tower interaction [1]. Due to the simplistic design of the model wind turbine case in the current simulations (as discussed in Section 2.3.4), the observations regarding the BTI noise generating mechanism cannot be extrapolated to an industrial wind turbine. ...
... For both separation distances, the contribution from the tower is dominant and increases with frequency. Such a spectral shape in the infrasonic frequency region is commonly observed in the vicinity of modern industrial wind turbines [1,31] with its hypothesized origin to be the blade tower interaction [1]. Due to the simplistic design of the model wind turbine case in the current simulations (as discussed in Section 2.3.4), the observations regarding the BTI noise generating mechanism cannot be extrapolated to an industrial wind turbine. ...
This paper describes the generation of blade–tower interaction (BTI) noise from upwind turbines and pylon-mounted fans using a combination of experimental and numerical means. An experimental rotor-rig was used in an anechoic chamber to obtain BTI acoustic data under controlled conditions. A computational model, based on the solution of the unsteady Reynolds Averaged Navier Stokes (URANS) equations and Curle's acoustic analogy, was used to describe the generation of fan and simplistic model of wind turbine BTI noise by the rotor-rig. For both the fan and model wind turbine case, the tower was found to be a more significant source of BTI noise than rotor blades. The acoustic waveforms for both turbine and fan are similar; however, in the case of the turbine, the blade contribution reinforces that from the tower, while in the case of a fan, there is some cancellation between the tower source and the blade source. This behavior can be explained by the unsteady aerodynamics occurring during BTI.
... Acoustic measurements in the low-frequency range 3.3 km from a wind farm show discrete peaks at the bladepassing frequency (BPF) and its higher harmonics below 20 Hz (Hansen et al., 2017). This was also observed by Pilger and Ceranna (2017), who evaluated the data obtained by a micro-barometer array for infrasound detection located in northern Germany. Zajamšek et al. (2016) compared outdoor and indoor measurements close to an Australian wind farm and found the same tonal character in the noise spectra. ...
... The results of both studies coincide with the findings of the presented paper where tower base loads at BPF harmonics close to eigenfrequencies of the turbine are prominent in the spectra. The tonal char-acter of the low-frequency noise was also shown in acoustic field measurements (Hansen et al., 2017;Pilger and Ceranna, 2017). They showed that the BPF harmonics are dominant in the measured spectra and thus the peak frequencies shift depending on the rotational speed of the turbine. ...
... They showed that the BPF harmonics are dominant in the measured spectra and thus the peak frequencies shift depending on the rotational speed of the turbine. Pilger and Ceranna (2017) furthermore compared measurements of a single 200 kW turbine to estimated SPL from the Viterna method (Viterna, 1981). They found an underestimation of SPL which they explained with environmental conditions neglected in the model. ...
The low-frequency emissions from a generic 5MW wind turbine are investigated numerically. In order to regard airborne noise and structure-borne noise simultaneously, a process chain is developed. It considers fluid–structure coupling (FSC) of a computational fluid dynamics (CFD) solver and a multi-body simulations (MBSs) solver as well as a Ffowcs-Williams–Hawkings (FW-H) acoustic solver. The approach is applied to a generic 5MW turbine to get more insight into the sources and mechanisms of low-frequency emissions from wind turbines. For this purpose simulations with increasing complexity in terms of considered components in the CFD model, degrees of freedom in the structural model and inflow in the CFD model are conducted. Consistent with the literature, it is found that aeroacoustic low-frequency emission is dominated by the blade-passing frequency harmonics. In the spectra of the tower base loads, which excite seismic emission, the structural eigenfrequencies become more prominent with increasing complexity of the model. The main source of low-frequency aeroacoustic emissions is the blade–tower interaction, and the contribution of the tower as an acoustic emitter is stronger than the contribution of the rotor. Aerodynamic tower loads also significantly contribute to the external excitation acting on the structure of the wind turbine.
... It should be borne in mind that propagation of nonlinear infrasound waves ranges from 17 Hz to 340 Hz and there are very poorly damped and, therefore, can travel over relatively long distances. In addition, apart from sources of infrasonic signals such as road vehicles, passing trains or agricultural machines, which should be taken into account when measurements are performed, there may be other more natural sources of their emission, such as thunder, waves on the water surface, wind gusts at high speeds meeting obstacles (sweeping phenomenon), etc. [6][7][8][9]. In the case of wind farms comprising many WT, which can additionally be varied in terms of design, capacity and service life, the varied nature of generated infrasound should be taken into account when measurements are performed and assessment is made of the obtained results. ...
... Usually, WT are located at such distances apart that they do not interfere in the aerodynamic conditions of their operation; however, the generated acoustic noise in the low-frequency signal range has not been comprehensively analyzed so far. The results of many years of investigations concerned with the influence of periodic wind turbine noise on infrasound array measurements are reported in [8]. Katinas et al. [15] showed that the level of infrasound generated by a wind turbine can be comparable to the background level even when the wind speed is 12 m/s, which provides optimal working conditions by most wind turbines currently available on the market. ...
The research reported in this paper involves the development and refinement of methods applicable to the measurement and analysis of infrasound signals generated by the operation of wind turbines. In particular, the presentation focuses on the use of a new system that is applied for simultaneous recording of acoustic signals in the low-frequency range emitted by wind farms in three independent and identical measurement setups. A comparative analysis of the proposed new system was made with the Brüel & Kjaer measurement, a commonly used methodology, which meets the requirements of the IEC 61400-11 standard. The paper focuses on the results of frequency and time-frequency analysis of infrasound signals recorded throughout the operation of a wind turbine with a rated capacity of 2 MW. The use of a correlated system with three simultaneous measurement systems can be a new and alternative measurement method that will eliminate the drawbacks of previous approaches.
... However, due to the impulsive nature of BTI noise, the acoustic spectra typically consist of a tonal peak at the BPF accompanied by additional peaks at its higher harmonics. [3][4][5] The high levels of the harmonics suggests that although the BPF is in the infrasound range, the harmonic tones may be within the audible frequency range of human hearing. ...
... The existence of BTI noise has been noted in previous experimental studies. 3,4,6 A number of numerical studies on BTI noise have also been conducted, but they tend to focus only on the mechanism of the noise from blade due to the modified flow field. [7][8][9] While blade passage effects on the aerodynamic performance have been widely researched, [10][11][12] it is only recently that its effects on BTI noise production were investigated by Yauwenas et al. 13 and Zajam sek et al. 2 These recent studies on BTI noise 2,13 performed combined experimental and numerical studies on a simplified wind turbine model, where the distance between the blade and the tower, d, varies from d=D ¼ 2=7 to 1, where D is the rotor diameter. ...
This paper presents a combined experimental and numerical study that characterises the directivity of blade-tower interaction (BTI) noise. Numerical computations were performed using a hybrid approach combining unsteady Reynolds-averaged Navier-Stokes equations and Curle's acoustic analogy, allowing the noise from the blades and the tower to be computed separately. The noise directivity of the blade and the tower components have a dipole pattern and a monopole-like pattern, respectively; hence, the resulting BTI noise directivity resembles an oval. Partial cancellations between the blade and tower components are also shown to affect the BTI noise directivity.
... The dynamic development of wind farms directly contributes to the growing interest among local communities, local government, environmental organizations, or governing elites in conducting a objective evaluation of the possible effects of their effect on the natural environment, including in particular in making an evaluation of their impact on the health of people living in the areas adjacent to wind power plants. One of important issues, more and more often raised by various kinds of institutions or authorities issuing opinions on their potential effect and effect on the environment, is the acoustic noise generated by contemporary wind turbines [1][2][3][4][5][6]. ...
... In addition, there are no uniform legal regulations, which are additionally subject to continuous changes and modifications. This applies, first of all, to signals in the low frequency range (1 Hz-250/300 Hz), including in particular infrasounds (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20), where there are often no legal regulations at all, and, to a smaller extent, the audible range (16)(17)(18)(19)(20). However, it should be noted, that at sufficiently high levels of sound intensity, infrasound can be heard by humans, which may be the case when it is generated by industrial installations [7]. ...
The subject matter of the paper is concerned with improvement in the methods of measuring, processing and analyzing the infrasound noise caused by working wind turbines. The article presents results of measurements and analyses of infrasound signals emitted by work of a wind turbine with 2 MW nominal power, during its normal operation. Infrasounds were recorded at the same time in three independent and identical, in terms of the applied devices, measurement tracks. For the measurements of infrasound signals a dedicated measurement system was used. The measurements were made at variable weather conditions and for different distances between the measurement points and the turbine being examined. In the results part of the paper the following are subsequently presented: the time courses of the recorded infrasound signals, and then courses of the periodograms designated for them in frequencies and the spectrograms calculated for time-frequency data. Special attention was paid to the analysis of the possible application of time-frequency ridge transformation, the application of which enables effective extraction of energy-wise dominant instantaneous frequencies. In the summary, an analysis was made of the results obtained by means of frequency, time-frequency and time-frequency ridge analysis methods.
... The vibration emissions are linked to the excited eigenmodes of the WT towers (Nagel et al. 2021 and occur mainly at frequencies below 10 Hz. It is reported that the expansion of nearby wind energy installations reduces the quality of gravity wave observations (Saccorotti et al. 2011) or seismological recordings at many sites (Neuffer and Kremers 2017, Pilger and Ceranna 2017, Stammler and Ceranna 2016, Zieger and Ritter 2018. As a consequence, (micro-) earthquake detection capabilities are reduced for state earthquake services when more and more recording sites are surrounded by an increasing number of WTs. ...
The characterization and prediction of wind turbine (WT) emissions are important steps in reducing their impact on humans or sensitive technologies such as seismic stations or physics experiments. Here, WT ground motion emissions are studied along two measurement lines set up at two wind farms on the Eastern Swabian Alb, southwest Germany. The main purpose of the data analysis is to estimate amplitude decay rates from vertical component data and surface wave phase velocities excited by the permanent motion of the WT towers. Phase velocities as well as geological information serve as input to build realistic subsurface models for numerical wave field simulations. Amplitude A decay rates are characterized by b -values through A ∼ 1 / r b depending on distance r and are derived from peaks in power spectral density (PSD). We find an increase of b PSD with frequency from 0.5 to 3.2 for field data. For low frequencies (1.2 Hz and 3.6 Hz), b PSD ranges from 0.5 to 1.1, hence close to the geometrical spreading factor of surface waves ( b PSD = 1 ). Anelastic damping and scattering seem not to be significant at these frequencies which also shows in numerical simulations for quality factors Q = 50 - 200 . We also find that the emitted wavefields from several WTs interfere, especially in the near-field, and produce strong local ground motion amplitudes. The inclusion of a steep topography present in low mountain ranges adds more wave field distortions which can further increase the amplitudes. This needs to be considered when predicting WT induced ground motions.
... The larger the rotor, the lower the emitted frequency. Infrasound of IWT's is meanwhile with big parts in the range of 0.1 to 10 Hz according to [44] [45]. Its infrasound emission is impulsive in the effect/time profile according to Roos W, Vahl, CF (2021) [45] and Vanderkooy [46], as shown in Figure 6. ...
... In this regard, the procedures described in the PN-EN 61400-11:2004/A1 standard are most often used, which, however, are applicable to measurements performed in the audible band and only offer its application for infrasound signals emitted by wind turbines. As a result, there is an objective difficulty in the comparative analysis of the results obtained by various research centers, which in turn makes it impossible to verify them and to formulate more comprehensive conclusions (Carlile et al., 2018;Freiberg et al., 2019;Hansen and Hansen, 2020;Heinzel et al., 2002;H€ ubner et al., 2019;van Kamp and van den Berg, 2018;Klein et al., 2018;Møller and Pedersen, 2004;Pilger and Ceranna, 2017;Shepherd and Hubbard, 1991;Stead et al., 2014;Yauwenas et al., 2017;Zajam sek et al., 2016). Additionally, there is no practical possibility to recreate the performed measurements and, above all, to perform adequate transformations of the recorded infrasound signals and their analysis, as well as to determine the repeatability of the results. ...
The issues reported in this article concern the development of methods applied for measurement, processing, and analysis of infrasound signals generated in association with the operation of wind farms. In particular, the discussion involves the results of the analysis using synchrosqueezed wavelet transforms of infrasound noise emitted by a 2 MW wind turbine that have been recorded during its operation in actual conditions. To record infrasound signals, a wireless measurement system was used, consisting of a base station and three synchronized mobile recording stations. To identify the wavelet structures with the highest ratio of energy, the synchrosqueezed wavelet transforms were used, and the courses of six time runs representing instantaneous frequencies were determined. Application of this approach enables the selection of energy-dominant waveforms from the time-frequency images, whose assessment can be performed mainly in terms of qualitative measures. Application of the synchrosqueezed wavelet transform is an effective tool for the purposes of detection and selection in the designated wavelet structures for the recorded infrasound dominant frequencies for which the carried energy ranges have the highest value.
... In fact, human hearing does not stop at 20 Hz, but the perception of pitch is lost and sensitivity lowers as the frequency drops. According to various sources, the range of infrasound frequencies is from 0.1 to 20 Hz and also includes audible low frequencies [7][8][9][10][11][12]. ...
The development of wind energy and the increasing number of installed wind turbines make it necessary to assess them in terms of the nuisance of the emitted infrasound noise generated by such devices. The article presents the results of measurements and analyses of infrasound emitted during the operation of wind turbines installed in various locations in Poland. Comparative analysis of noise levels in the infrasound and audible range has shown that acoustic energy is mainly in the low and infrasound frequency range, and the measured levels depend significantly on the weighting curves used. On the basis of the results, it was confirmed that the sound pressure level of infrasound signals emitted by the operation of high-power wind turbines, regardless of wind velocity, weather conditions, design solutions of turbines, operating time, rated capacity, does not exceed the criteria specified in the applicable legislation dealing with the assessment of infrasound noise on the working environment.
... Relatively poor damping, wavelength and frequency, combined with the possibility of standing waves that can be formed in field specific conditions, as well as the possible resonance phenomenon lead to objective difficulties in the unambiguous and adequate location of the source of infrasound generation. An important element is also related with the need to take into account the acoustic background during the measurements of infrasound emitted by wind turbines, the level of which, in many cases, may be close to the useful signals [4][5][6][7][8]. In particular, this applies to the conditions when wind speeds with values above 12-15 m/s occur during infrasound recording. ...
The study reported in this paper is concerned with areas related to developing methods of measuring, processing and analyzing infrasound noise caused by operation of wind farms. The paper contains the results of the correlation analysis of infrasound signals generated by a wind turbine with a rated capacity of 2 MW recorded by three independent measurement setups comprising identical components and characterized by the same technical parameters. The measurements of infrasound signals utilized a dedicated measurement system called INFRA, which was developed and built by KFB ACOUSTICS Sp. z o.o. In particular, the scope of the paper includes the results of correlation analysis in the time domain, which was carried out using the autocovariance function separately for each of the three measuring setups. Moreover, the courses of the cross-correlation function were calculated separately for each of the potential combinations of infrasound range recorded by the three measuring setups. In the second stage, a correlation analysis of the recorded infrasound signals in the frequency domain was performed, using the coherence function. In the next step, infrasound signals recorded in three setups were subjected to time-frequency transformations. In this part, the waveforms of the scalograms were determined by means of continuous wavelet transform. Wavelet coherence waveforms were calculated in order to determine the level of the correlation of the obtained dependencies in the time-frequency domain. The summary contains the results derived from using correlation analysis methods in the time, frequency and time-frequency domains.
... In this section, we study the vibrations in a Solar TITAN 130 gas turbine based on the actual data recorded by the monitoring system and its behavior following a critical start-up situation.The vibrations studied were recorded on turbine No. 01, when it was started, these temporal vibration variations indicated in Figures 6,9,10,11,12 and 13 are obtained by the vibration monitoring system of the turbine, they showed that the vibrations of the turbine TITAN 130 have the following characteristics; Figures 12 and 13 show that the type of random vibration of the pulse type and the amplitude of the vibrations is shown in Figures 9, 10 and 11 is variable over time and depends on the speed of the turbine (turbine speed represented in Figure 3 and velocity vibration (velocity of vibration) is important when starting the turbine and decreases relatively with the stability of the speed of the NGP turbine is shown in Figure 8. ...
Vibration monitoring is still a hot topic in most industrial sectors, using rotating machines. This domain is designed to provide protection measures against these instability phenomena and provides the same level of information for their diagnostics and the evaluation of the thorough dynamic behavior performed on these rotating machines. However, in gas turbines vibratory broadband signals are often difficult to interpret due to problems with the reliability of the data collected on these gas turbines in the environment of their farms. Given the costs of their maintenance, it is a good idea to strike a good balance between their preventive maintenance and corrective maintenance. This balance is ensured by the continuous monitoring of this type of machine, in order to act quickly and in a timely manner to avoid serious incidents. In this work, a strategy to monitor the vibrations of a Solar TITAN 130 gas turbine based on an automated diagnostic system is proposed to increase the monitoring capacity of the vibration behavior of the gas turbine being examined.
... Feature 2 in the XC is modeled as a variation in bulk air temperature that produces a small change in effective velocity. Adapted from Marcillo et al. (2014) used for interferometric studies in the lower atmosphere (Marcillo et al. 2015;Pilger and Ceranna 2017). Coda wave interferometry (Snieder 2006) can also be applied to acoustic waveforms from the same source-sensor pair (assuming a discrete repeating source) to estimate changes in atmospheric conditions. ...
Infrasound recordings can be used as input to inversion procedures to delineate the vertical structure of temperature and wind in a range of altitudes where ground-based or satellite measurements are rare and where fine-scale atmospheric structures are not resolved by the current atmospheric specifications. As infrasound is measured worldwide, this allows for a remote sensing technique that can be applied globally. This chapter provides an overview of recently developed infrasonic remote sensing methods. The methods range from linearized inversions to direct search methods as well as interferometric techniques for atmospheric infrasound. The evaluation of numerical weather prediction (NWP) products shows the added value of infrasound, e.g., during sudden stratospheric warming (SSW) and equinox periods. The potential transition toward assimilation of infrasound in numerical weather prediction models is discussed.
... The results of both studies coincide with the findings of the presented paper where tower base loads at BPF harmonics close to eigenfrequencies of the turbine are prominent in the spectra. The tonal character of the low-frequency noise was also shown in acoustic field measurements (Hansen et al., 2017;Pilger and Ceranna, 2017). They showed that the BPF harmonics are dom-5 inant in the measured spectra and thus the peak frequencies shift depending on the rotational speed of the turbine. ...
The low-frequency emissions from a generic 5MW turbine are investigated numerically. In order to regard airborne noise and structure-borne noise simultaneously a process chain was developed. It considers fluid-structure coupling (FSC) of a computational fluid dynamics (CFD) solver and multibody simulations (MBS) solver as well as a Ffowcs Williams-Hawkings (FW-H) acoustic solver. The approach was applied to a generic 5MW turbine to get more insight into the sources and mechanisms of low-frequency emissions from wind turbines. For this purpose simulations with increasing complexity in terms of considered components in the CFD model, degrees of freedom in the structural model and inflow in the CFD model were conducted. Consistent with literature, it has been found that aeroacoustic low-frequency emission is dominated by the blade-passing frequency harmonics. The tower base loads, which excite seismic emission, tend to be dominated by structural eigenfrequencies with increasing complexity of the model. The main source of aeroacoustic emissions is the blade-tower interaction and the contribution of the tower as an acoustic emitter is stronger than the contribution of the rotor. Aerodynamic tower loads also significantly contribute to the external excitation acting on the structure of the wind turbine.
... Consequently, rocket launches, military and industrial blasts, volcanic activity and meteoroid entries are among the strongest sources of infrasound (Mc Laughlin et al. 2000;Stevens et al. 2002;Ceranna et al. 2009;Matoza et al. 2011;Marchetti et al. 2013;Le Pichon et al. 2013;Pilger et al. 2015). Other, less intense infrasound signatures are generated by supersonic flights, microbaroms, severe weather, mining activity and wind turbines (Le Pichon et al. 2002;Garcés et al. 2004;Sindelarova et al. 2009;Gibbons et al. 2015a;Pilger and Ceranna 2017). This study focuses on anthropogenic infrasound signatures and the main interest therefore is on high-frequency (HF) infrasound above 0.7 Hz. ...
The European Infrasound Bulletin highlights infrasound activity produced mostly by anthropogenic sources, recorded all over Europe and collected in the course of the ARISE and ARISE2 projects (Atmospheric dynamics Research InfraStructure in Europe). Data includes high-frequency (> 0.7 Hz) infrasound detections at 24 European infrasound arrays from nine different national institutions complemented with infrasound stations of the International Monitoring System for the Comprehensive Nuclear-Test-Ban Treaty (CTBT). Data were acquired during 16 years of operation (from 2000 to 2015) and processed to identify and locate ∼ 48,000 infrasound events within Europe. The source locations of these events were derived by combining at least two corresponding station detections per event. Comparisons with ground-truth sources, e.g., Scandinavian mining activity, are provided as well as comparisons with the CTBT Late Event Bulletin (LEB). Relocation is performed using ray-tracing methods to estimate celerity and back-azimuth corrections for source location based on meteorological wind and temperature values for each event derived from European Centre for Medium-range Weather Forecast (ECMWF) data. This study focuses on the analysis of repeating, man-made infrasound events (e.g., mining blasts and supersonic flights) and on the seasonal, weekly and diurnal variation of the infrasonic activity of sources in Europe. Drawing comparisons to previous studies shows that improvements in terms of detection, association and location are made within this study due to increasing the station density and thus the number of events and determined source regions. This improves the capability of the infrasound station network in Europe to more comprehensively estimate the activity of anthropogenic infrasound sources in Europe.
... Noise is unwanted sound from activity in certain rate and time, which can cause human health problems and disturbs environmental comfort [1 and 2], Turbo-compressor (TC) are well known machines with a development of near a hundred years, Centrifugal compressors of industrial systems use mechanical energy to compress the working fluid [3]. Nowadays, there is a great concern about noise in city-based and industrial areas [4][5][6][7][8][9][10][11][12][13][14][15][16][17] The noise generated from operating wind and gas turbines [18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35] is classifiable as an aerodynamic or a mechanical sound as a whole. Aerodynamic sound is generated from the blade passing through the air, and mechanical sound is emitted by some equipment in the nacelle [36]. ...
The fundamental aim of this study is to compare between the noise generated by the
moto-compressor and the noise generated by the turbo-compressor operating 24H/24H on the
continuous function mode; these two machines make part of the equipment of the GP1Z, a
factory of hydrocarbon treatment. To attain the principal objective of this study we divided
our work into two parts, in the first part we followed and evaluated the average level of the
noise emitted by the two machines, whereas in the second part we studied the noise
propagation emitted by the two machines and its impact on the generation of the noise. The
results obtained from this study demonstrate that the noise generated by the turbo-compressor
is higher than the noise generated by the moto-compressor.
Keywords: noise; moto-compressor; turbo-compressor; noise maping.
... At night hours, the cooling of the ground is faster than the corresponding one in the atmosphere causing higher sound speeds and downward bending of the propagation direction of infrasound. A plethora of published papers and reports can be found on the subject, some representatives are those of Manley et al. [19], Møller and Pedersen [24], [39], Öhlund and Larsson [26], Jakobsen [12], Turnbull et al. [38], Kelley et al. [15], Zajamšek et al. [46], Hoffmeyer and Jakobsen [10], Pedersen et al. [27], Keith et al. [14], Carman [7], Michaud et al. [21,22], Marcillo et al. [20], Katinas et al. [13] and Pilger and Ceranna [28]. ...
Infrasound, low frequency noise and soil vibrations produced by large wind turbines might disturb the comfort of nearby structures and residents. In addition repowering close to urban areas produces some fears to the nearby residents that the level of disturbance may increase. Due to wind loading, the foundation of a wind turbine interacts with the soil and creates micro-seismic surface waves that propagate for long distances and they are able to influence adversely sensitive measurements conducted by laboratories located far from the excitation point. A numerical study on the creation and propagation of those waves to the surrounding area is the subject of the present work. Besides, the contribution of those waves to airborne sound generated by the soil-air interaction is also investigated. All numerical simulations are performed with the aid of the Boundary Element Method (BEM), which is ideal for solving such problems since it takes automatically into account the radiation conditions of the waves and thus only the soil-foundation interface and the free surface of the surrounding soil are needed to be discretized. Foundation and soil are considered as linearly elastic materials with interfacial bonding. The frequency domain Helmholtz equation is employed for the simulation of acoustic waves. Numerical results dealing with the airborne and soil borne noise propagation and attenuation are presented and disturbances that might be caused to nearby and far-field structures are discussed.
With the continuous warming of the global climate, expanding the use of renewable energy has become one of the main social responsibilities. However, as the number of installed wind turbines and their physical dimensions continue to increase, the issue of generated noise has become increasingly significant in influencing the acceptance and endorsement of wind power projects by neighboring communities. In this paper, we investigated the noise generated by two wind turbine units with rated powers of 1.5 MW and 4.5 MW and analyzed the variations in low-frequency noise during their operation and shutdown periods. This research shows that the power of a single unit has a significant impact on the low-frequency noise emitted into the environment. Compared with 1.5 MW wind turbines, 4.5 MW wind turbines generate more low-frequency noise when operating at the same wind speed. Further analysis of the narrowband frequency spectra and one-third octave spectra of the measured noise indicates that the low-frequency noise from the 4.5 MW wind turbine increases significantly in the range of approximately 80 Hz to 300 Hz, with more pronounced variations below 250 Hz corresponding to changes in wind speed. However, the overall variations in low-frequency noise with wind speed are not as notable as those observed for the 1.5 MW wind turbine. Due to the relatively weak attenuation of low-frequency noise in the atmosphere, the higher low-frequency content of large wind turbines may cause more distress to residents near wind farms. The result of this study emphasizes that in the planning and design of wind power projects, in addition to considering the efficiency of single-unit power generation and the contribution of renewable energy, it is also necessary to pay full attention to noise emission issues to ensure that the project is widely supported and accepted in the community.
The characterization and prediction of wind turbine (WT) emissions are important steps in reducing their impact on humans or sensitive technologies such as seismic stations or physics experiments. Here, WT ground motion emissions are studied along two measurement lines set up at two wind farms on the Eastern Swabian Alb, southwest Germany. The main purpose of the data analysis is to estimate amplitude decay rates from vertical component data and surface wave phase velocities excited by the permanent motion of the WT towers. Phase velocities as well as geological information serve as input to build realistic subsurface models for numerical wave field simulations. Amplitude A decay rates are characterized by b-values through A~1/rb depending on distance r and are derived from peaks in power spectral density (PSD). We find an increase of bPSD with frequency from 0.5 to 3.2. For low frequencies (1.2 Hz and 3.6 Hz), bPSD~0.5-1.1, hence close to the geometrical spreading factor of surface waves (bPSD=1). Anelastic damping and scattering seem not to be significant at these frequencies which also shows in numerical simulations for quality factors Q=50-200. We also find that the emitted wavefields from several WTs interfere, especially in the near-field, and produce strong local ground motion amplitudes. The inclusion of a steep topography present in low mountain ranges adds more wave field distortions which can further increase the amplitudes. This needs to be considered when predicting WT induced ground motions.
The record of seismic, hydroacoustic, and infrasonic waves is essential to detect, identify, and localize sources of both natural and anthropogenic origin. To guarantee traceability and inter-station comparability, as well as an estimation of the measurement uncertainties leading to a better monitoring of natural disasters and environmental aspects, suitable measurement standards and reliable calibration procedures of sensors, especially in the low-frequency range down to 0.01 Hz, are required. Most of all with regard to the design goal of the Comprehensive Nuclear-Test-Ban Treaty Organisation’s International Monitoring System, which requires the stations to be operational nearly 100% of the time, the on-site calibration during operation is of special importance. The purpose of this paper is to identify suitable excitation sources and elaborate necessary requirements for on-site calibrations. We give an extensive literature review of a large variety of anthropogenic and natural sources of seismic, hydroacoustic, and infrasonic waves, describe their most prominent features regarding signal and spectral characteristics, explicitly highlight some source examples, and evaluate the reviewed sources with respect to requirements for on-site calibrations such as frequency bandwidth, signal properties as well as the applicability in terms of cost–benefit. According to our assessment, earthquakes stand out across all three waveform technologies as a good natural excitation signal meeting the majority of the requirements. Furthermore, microseisms and microbaroms allow a calibration at very low frequencies. We also find that in each waveform technique man-made controlled sources such as drop weights or air guns are in good agreement with the required properties, although limitations may arise regarding the practicability. Using these sources, procedures will be established allowing calibration without record interrupting, thereby improving data quality and the identification of treaty-related events.
Narrowband harmonic infrasound signals within the 1 to 8 Hz passband, generated by wind turbines on Ascension Island, have been recorded at four microbarometers located at distances of between 1.8 and 4.6 km from the source along different azimuths. Across one month of recordings in October 2010, amplitude ratios between the four recordings show temporal stability but deviate from the ratios expected for propagation across a flat plane. Using a recently developed three-dimensional parabolic equation method, that can incorporate realistic topography as a lower boundary, it is shown that these time-independent amplitude ratio deviations can be, in part, explained by acoustic interactions with topography that has scale lengths on the order of a few hundreds of metres. These interactions comprise both two-dimensional barrier effects that reduce sound levels behind high topography, and three-dimensional diffractive effects that increase sound levels behind topographic obstacles. For the Ascension Island case study, amplitudes along two of the four paths can be successfully modelled using a two-dimensional model, indicating that barrier effects dominate for these path geometries. Amplitude ratios along a third path, and the frequency-dependence of these ratios, are better simulated using a three-dimensional model that captures the out-of-plane diffractive effects around a prominent hill. The fourth path is poorly modelled using the three-dimensional model, which overpredicts acoustic amplitudes in this case. We hypothesise that this mismatch is likely to be due to a simplified description of the wind turbine source term. This study provides further observational confirmation that topographic interactions need to be considered when interpreting locally propagating infrasound, and shows that for harmonic narrowband sources a parabolic equation solver incorporating realistic boundary conditions provides an efficient method for simulating topographic interactions.
The infrasonic sound pressure levels due to the blade-tower interaction generated by a wind turbine in the publication by Pilger and Ceranna (JSV, Vol. 388, pp. 188–200, 2017) have been miscalculated with a constant, systematic offset in the order of 36 dB for the sound pressure levels. All values were originally derived from power spectral densities, which will exclusively be considered for presenting corrected figures and results in agreement with Baumgart et al. (JSV, doi: 10.1016/j.jsv.2021.116310, 2021). We also provide further explanation regarding the electrical power of the studied wind turbine. Since the miscalculation is of a systematic nature, the results of our study in terms of estimating distance ranges in which infrasound generated by wind turbines is detected by highly sensitive microbarometers, are still valid. Nevertheless, the findings in our study are inappropriate for drawing any conclusions on the effect of infrasound from wind turbines on human beings.
A subset of seismic signals generated in industrial environments displays spectral peaks organized in sequences of fundamental frequencies with multiple overtones, which we refer to as tonal noise (TN). Using one year of data from each of 1732 stations in the USArray Transportable Array, we detected around 1.5 million TN sequences in the contiguous United States, which corresponds (on average) to around 2.4 detections per day (869 detections per year) at each station. TN across the continent is clustered around specific regions and frequencies. The majority (>70%) of stations in the 90th percentile of total detection numbers (more than 2100 detections per year) are concentrated in the Interior Plains, Canadian Shield, and Appalachian Highlands. We found that the fundamental frequencies of all TN detections are concentrated in six spectral bands with value ranges of 0.9–0.95, 1.8–1.85, 2.5–2.55, 3.3–3.35, 5–5.05, and 5.45–5.5 Hz with around 104, 37, 46, 37, 62, and 45 thousand detections, respectively. Detections in these bands account for around 22% of all detections. We suggest that large regions with similar TN are related to noise from industrial activities driven by physiographic characteristics such as favorable winds or abundance of water (wind and hydroelectric power generators). The presence of TN and other spectrally discrete components in the seismic wavefield is a ubiquitous feature in the seismic background. This type of noise has the potential to affect subsurface imaging efforts by introducing potentially static and continuous sources of noise. The effects of TN can be especially significant as near-surface imaging studies move toward utilizing higher frequency (>1 Hz) for ambient seismic noise.
This work deals with the phenomena of vibration of turbines in accidental situations, the appearance and propagation of a crack or hole in a blade of a wheel of the rotating machine is a phenomenon that must also be taken into account Both by the manufacturers and by the operators of these machines. In order to ensure that their installations are not damaged, the latter have a few tools in which online vibration monitoring is one of the most commonly used. Based on a vibration measurement at bearings, it allows continuous monitoring of the mechanical condition of the equipment and an early diagnosis of the main defects. Firstly, we study the system of vibration monitoring of the turbine. From the regularly collected vibrations, it is possible to detect any malfunctions and to follow their evolution in order to plan or postpone a mechanical intervention. This continuous monitoring is carried out during operation of the machine.
Detecting a Signal Of Interest (SOI) is the first step in many applications of infrasound monitoring. This intuitively simple task is defined as separating out signals from background noise on the basis of the characteristics of observed data; it is, however, deceptively complex. The problem of detecting signals requires multiple processes that are divisible at their highest level into several fundamental tasks. These tasks include (1) defining models for SOIs and noise that properly fit the observations, (2) finding SOIs amongst noise, and (3) estimating parameters of the SOI (e.g., Direction Of Arrival (DOA), Signal-to-Noise Ratio (SNR) and confidence intervals) that can be used for signal characterization. Each of these components involves multiple subcomponents. Here, we explore these three components by examining current infrasound detection algorithms and the assumptions that are made for their operation and exploring and discussing alternative approaches to advance the performance and efficiency of detection operations. This chapter does not address new statistical methods but does offer some insights into the detection problem that may motivate further research.
We present a case study on the detection and quantification of seismic signals induced by operating wind turbines (WTs). We spatially locate the sources of such signals in data which were recorded at 11 seismic stations in 2011 and 2012 during the TIMO project (Deep Structure of the Central Upper Rhine Graben). During this time period, four wind farms with altogether 12 WTs were in operation near the town of Landau, Southwest Germany. We locate WTs as sources of continuous seismic signals by application of seismic interferometry and migration of the energy found in cross-correlograms. A clear increase of emitted seismic energy with rotor speed confirms that the observed signal is induced by WTs. We can clearly distinguish wind farms consisting of different types of WTs (different hub height and rotor diameter) corresponding to different stable frequency bands (1.3–1.6 Hz, 1.75–1.95 Hz and 2.0–2.2 Hz) which do not depend on wind speed. The peak frequency apparently is controlled by the elastic eigenmodes of the structure rather than the passing of blades at the tower. From this we conclude that vibrations are coupled into the ground at the foundation and propagate as Rayleigh waves (and not as infrasound). The migration velocity of 320 m/s corresponds to their group velocity. The applied migration method can contribute to the assessment of local sources of seismic noise. This topic gets growing attention in the seismological community. In particular, the recent boost of newly installed wind farms is a threat to seismological observatories such as the Black Forest Observatory (BFO) and the Gräfenberg array (GRF) or gravitational wave observatories (e.g. LIGO, VIRGO) in terms of a sensitivity degradation of such observatories.
This paper compares experimental and numerical data concerning the unsteady flowfield during blade-tower interaction (BTI) of a laboratory-scale fan. Experiments were conducted using a model scale, pylon-mounted fan rig. Wake velocity data over various planes and streamwise positions were obtained using hot wire anemometry, with a single-wire probe mounted to a traverse. A new phase-averaging procedure was developed to azimuthally decompose the experimental data. The phase averaging allowed better comparison with the numerical data at various values of blade azimuth. Numerical simulations were performed using an unsteady, Reynolds averaged Navier Stokes (URANS) approach using second-order numerical discretisation. In general, the comparison of the numerical model with the experimental data is good, giving confidence that the numerical model captures the important noise generating physics of the flow.
Sound from wind turbines involves a number of sound production mechanisms related to different interactions between the turbine blades and the air. An important contribution to the low frequency part of the sound spectrum is due to the sudden variation in air flow which the blade encounters when it passes the tower: the angle of attack of the incoming air suddenly deviates from the angle that is optimized for the mean flow. Hitherto, low-frequency sound from wind turbines has not been shown to be a major factor contributing to annoyance. This seems reasonable as the blade passing frequency is of the order of one hertz where the human auditory system is relatively insensitive. This argument, however, obscures a very relevant effect: the blade passing frequency modulates well audible, higher-frequency sounds and thus creates periodic sound: blade swish. This effect is stronger at night because in a stable atmosphere there is a greater difference between rotor averaged and near-tower wind speed. Measurements have shown that additional turbines can interact to further amplify this effect. Theoretically the resulting fluctuations in sound level will be clearly perceptible to human hearing. This is confirmed by residents near wind turbines with the same common observation: often late in the afternoon or in the evening the turbine sound acquires a distinct ‘beating’ character, the rhythm of which is in agreement with the blade passing frequency. It is clear from the observations that this is associated to a change toward a higher atmospheric stability. The effect of stronger fluctuations on annoyance has not been investigated as such, although it is highly relevant because a) the effect is stronger for modern (that is: tall) wind turbines, and b) more people in Europe will be living close to these wind turbines as a result of the growth of wind energy projects.
An infrasound field campaign was performed in 2011/2012 utilizing six single infrasound sensors along the great circle path between a known infrasound ground-truth source (ARIANE 5 engine test facility, Lampoldshausen, Germany) and a nearby receiver (German infrasound array I26DE, Bavarian Forest) covering a distance of about 320km in total. The recordings gathered provide new insights in the infrasonic wave propagation at regional distances and, in particular, through the near-source shadow zone by comparing measured signals with modeling results. Infrasound propagation modeling utilizing ray-tracing and parabolic equation approaches is performed to explain detections and non-detections at certain ranges from the ground-truth source. Modeling results show a significant influence of small-scale atmospheric variations (of e.g. wind and sound speed) on the propagation pattern, resulting in varying tropospheric and stratospheric ducting behavior. An extensive set of gravity wave profiles was tested to investigate the influences of atmospheric dynamics on the infrasound wave field and improve the modeling results. The modeling is also applied to complementary detections obtained during the field campaign; namely one case associated with two potential, contemporaneous and closely spaced infrasound sources, is presented in detail. Propagation modeling is used to resolve the source ambiguity between a ground-based and a higher altitude source and give a strong preference with respect to the observed infrasonic signatures.
Contours of equal loudness were determined in the frequency range 2–63 Hz and the loudness range 20–100 phon. The loudness curves run almost parallel in the infrasonic frequency range and much closer than in the audio region. Infrasound only a few dB above the hearing threshold will therefore seem loud and possibly annoying. The subjects were 20 normal hearing students aged between 18 and 25, and the psychometric method was based on maximum-likelihood estimation of psychometric functions.
Infrasound is discussed in terms of what it actually is, how the media has dealt with it and what those with limited knowledge say about it. The perception of infrasound occcurs at levels higher than the levels produced by wind turbines and there is now agreement amongst acousticians that infrasound from wind turbines is not a problem. Statements on infrasound from objectors are considered and it is shown how these may have caused avoidable distress to residents near wind turbines and also diverted attention from the main noise source, which is the repeating sound of the blades interacting with the tower. This is the noise which requires attention, both to reduce it and to develop optimum assessment methods.
The explosive fragmentation of the 2013 Chelyabinsk meteorite generated a large airburst with an equivalent yield of 500 kT TNT. It is the most energetic event recorded by the infrasound component of the CTBT-IMS, globally detected by 20 out of 42 operational stations. This study performs a station-by-station estimation of the IMS detection capability to explain infrasound detections and non-detections from short to long distances, using the Chelyabinsk meteorite as global reference event. Investigated parameters influencing the detection capability are the directivity of the line source signal, the ducting of acoustic energy and the individual noise conditions at each station. Findings include a clear detection preference for stations perpendicular to the meteorite trajectory, even over large distances. Only a weak influence of stratospheric ducting is observed for this low-frequency case. Furthermore, a strong dependence on the diurnal variability of background noise levels at each station is observed, favoring nocturnal detections.
Setbacks for wind turbines have been established in many jurisdictions to address potential health concerns associated with audible noise. However, in recent years, it has been suggested that infrasound (IS) and low-frequency noise (LFN) could be responsible for the onset of adverse health effects self-reported by some individuals living in proximity to wind turbines, even when audible noise limits are met. The purpose of this paper was to investigate whether current audible noise-based guidelines for wind turbines account for the protection of human health, given the levels of IS and LFN typically produced by wind turbines. New field measurements of indoor IS and outdoor LFN at locations between 400 and 900 m from the nearest turbine, which were previously underrepresented in the scientific literature, are reported and put into context with existing published works. Our analysis showed that indoor IS levels were below auditory threshold levels while LFN levels at distances >500 m were similar to background LFN levels. A clear contribution to LFN due to wind turbine operation (i.e., measured with turbines on in comparison to with turbines off) was noted at a distance of 480 m. However, this corresponded to an increase in overall audible sound measures as reported in dB(A), supporting the hypothesis that controlling audible sound produced by normally operating wind turbines will also control for LFN. Overall, the available data from this and other studies suggest that health-based audible noise wind turbine siting guidelines provide an effective means to evaluate, monitor, and protect potential receptors from audible noise as well as IS and LFN.
Preterm infants born before the 30th week of pregnancy are especially at risk of perinatal brain damage which is usually a result of cerebral ischemia or an ascending intrauterine infection. Prevention of preterm birth and early intervention given signs of imminent intrauterine infection can reduce the incidence of perinatal cerebral injury. It has been shown that administering magnesium intravenously to women at imminent risk of a preterm birth leads to a significant reduction in the likelihood of the infant developing cerebral palsy and motor skill dysfunction. It has also been demonstrated that delayed clamping of the umbilical cord after birth reduces the rate of brain hemorrhage among preterm infants by up to 50%. In addition, mesenchymal stem cells seem to have significant neuroprotective potential in animal experiments, as they increase the rate of regeneration of the damaged cerebral area. Clinical tests of these types of therapeutic intervention measures appear to be imminent. In the last trimester of pregnancy, the serum concentrations of estradiol and progesterone increase significantly. Preterm infants are removed abruptly from this estradiol and progesterone rich environment. It has been demonstrated in animal experiments that estradiol and progesterone protect the immature brain from hypoxic-ischemic lesions. However, this neuroprotective strategy has unfortunately not yet been subject to sufficient clinical investigation.
The first work which described the harmonic tonal nature of vibrations from windfarms was carried out at St Breock's Down, Cornwall, UK and is described in Legerton et al (1996) and more fully in Snow, and Styles (1997). This has since raised concerns about the possible effect of wind farms on sensitive installations. Styles et al (2005) describe an extensive monitoring programme to characterise the low frequency vibration spectra produced by wind turbines of various types, both fixed and variable speed. They demonstrated that small but significant harmonic vibrations controlled by the modal vibrations of the towers and excited by blade passing, tower braking and wind loading while parked, can propagate tens of kilometres and be detected on broadband seismometers. This meant that protective measures had to be implemented to protect the International Monitoring System (IMS) seismic monitoring station located at Eskdalemuir in the Scottish Borders, the UK's contribution to the Comprehensive Test Ban Treaty (CTBT) verification regime which must be observed by international treaty. Over 2 GW of wind turbines were planned for this region and planning restrictions were imposed to control development because of the potential effects on the IMS station. Styles et al (2005) established that vibrations of concern in the 2 to 6 Hz band, while small, were critical for this monitoring. Propagation laws and an aggregate vibration budget were derived and calculated to aid planning and permit appropriate wind farm development. With increasing pressure to reduce carbon emissions through renewable energy contributions in the UK and especially Scotland, the budget has now been reached, with at least 2.5 GW of new wind developments are still in scoping and planning. It is therefore necessary to find a method to reduce the vibrations from new and existing
farms to achieve headroom for new developments.
Reactec Ltd in conjunction, have developed a Seismically Quiet Tower (SQT) system
which can be retro-fitted or installed during construction. This can significantly
attenuate the vibrations produced and delivered to the ground in the frequency band
deleterious to the discrimination capability of the Eskdalemuir station. The SQT can,
and is planned to, be fitted to existing close-in wind turbines to reduce their
contribution to the vibration budget and potentially release budget for new
development elsewhere in the 50 km zone of concern around Eskdalemuir. Keele
University have carried out a programme of modelling and seismic monitoring of this
system and have confirmed that it does significantly reduce the vibration spectrum in
the region of 2 to 6 Hz which has the added benefit of reducing many fatigue loads
on the turbine tower itself.
An infrasound field campaign was performed in 2011/2012 utilizing single infrasound sensors along the great circle path between a known ground-truth source (Ariane 5 engine test facility, Lampoldshausen, Germany) and a regional receiver (German infrasound array IS26, Bavarian Forest) covering a distance of roughly 320 km in total. The recordings gathered provide new insights in the infrasonic wave propagation at regional and near-source distances by comparing measured signals with modeling results within this study.
Ray-tracing and parabolic equation approaches are utilized to model infrasound propagation from the ground-truth source to the line profile sensors and explain the obtained detections and non-detections. Modeling and observation results are compared by estimating their amplitude, quantifying amplitude deviations and also considering observed and calculated travel times and celerities. Modeling results show a significant influence of small-scale atmospheric variations in effective sound speed profiles on the propagation pattern, which results in varying tropospheric and stratospheric ducting behavior. A large number of gravity wave profiles is tested to investigate the influences of atmospheric dynamics on the infrasound wave field and improve the modeling results. The modeling is furthermore applied to a case of two potential, contemporaneous and closely spaced infrasound sources. Propagation modeling is used here to resolve the source ambiguity between a ground-based and a higher altitude source giving a strong preference to the latter with respect to the observed infrasonic signatures.
The good agreement between modeling and observation results within this study successfully shows the benefit of applying infrasound propagation modeling to the validation of infrasound measurements, verification of ducting behavior and discrimination of infrasound sources.
On 15 February 2013, a large Earth-impacting fireball disintegrated over the Ural Mountains. This extraordinary event is, together with the 1908 Tunguska fireball, among the most energetic events ever instrumentally recorded. It generated infrasound returns, after circling the globe, at distances up to ~85,000 km, and was detected at 20 infrasonic stations of the global International Monitoring System (IMS). For the first time since the establishment of the IMS infrasound network, multiple arrivals involving waves that traveled twice round the globe have been clearly identified. A preliminary estimate of the explosive energy using empirical period-yield scaling relations gives a value of 460 kt of TNT equivalent. In the context of the future verification of the Comprehensive Nuclear-Test-Ban Treaty, this event provides a prominent milestone for studying in detail infrasound propagation around the globe for almost 3 days as well as for calibrating the performance of the IMS network.
The stratospheric infrasound duct, formed between Earth's surface and
altitudes of ˜50 km, is sensitive to spatiotemporal variations in
stratospheric wind speed and temperature. Infrasound recorded at long
range from the 2010 summit eruption of Eyjafjallajökull volcano,
Iceland, exhibits temporal variability correlated with diurnal
stratospheric solar tidal wind speed variations. Between 18 and 28 April
2010, signal observations at stations BKNI, U.K. (range, 1745 km), and
IS18, Greenland (range, 2285 km), exhibit prominent diurnal variations
in infrasonic amplitude, bandwidth, back azimuth, and apparent speed,
which we identify using the CLEAN spectral analysis algorithm for
unevenly sampled time series. Results of 3-D acoustic ray tracing
through operational atmospheric specifications indicate that tidal wind
speed variations (with amplitudes of ˜20 m/s) can generate diurnal
variations in the proportion of the acoustic wavefield propagating
within the stratospheric acoustic duct. Range-dependent meteorology is
required; propagation modeling using averaged meteorological profiles
fails to predict the leakage of acoustic energy out of the stratospheric
acoustic duct at times of low observed signal amplitudes. Ray tracing
correctly predicts the phase of the observed signal amplitude and
apparent speed variations. Diurnal variability in ducting, combined with
diurnal variations in ambient noise at the sensors, can explain the
observed signal bandwidth variations. Back azimuth variations (observed
only along the Eyjafjallajökull to BKNI path) are not predicted by
3-D ray tracing. Tidal variations have implications for models of
infrasound array network detection capability and for studies that
utilize amplitude and bandwidth measurements to make inferences about
the acoustic source.
A theoretical understanding of the mechanisms by which quarry blasts excite seismic waves is useful in understanding how quarry blast discriminants may be transported from one region to another. An experiment in Texas with well-placed seismic stations and a cooperative blasting engineer has shed light on some of the physical mechanisms of seismic excitation at short periods (0.1–3 Hz). Azimuthal radiation patterns of the 0.2–3 Hz Rayleigh and Love waves are diagnostic of two proposed mechanisms for non-isotropic radiation from quarry blasts. Observations show that the Love and Rayleigh wave radiation patterns depend upon the orientation of the quarry benches. Two possible mechanisms for non-isotropic radiation are (1) the lateral throw of spalled material and (2) the presence of the topographic bench in the quarry.
The spall of material can be modelled by vertical and horizontal forces applied to the free surface with time functions proportional to the derivative of the momentum of the spalled material. We use wavenumber integration synthetics to model the explosion plus spall represented by seismic moment tensor sources plus point forces. The resulting synthetics demonstrate that the magnitude of the SH (Love) compared with the SV (fundamental Rayleigh or Rg) in the short period band (0.5–3 Hz) may be explained by the spall mechanism. Nearly all of the available mass must participate in the spall with an average velocity of 2–5 m s−1 to provide sufficient impulse to generate the observed Love waves. Love wave radiation patterns from such a mechanism are consistent with the spall mechanism.
We modelled the effects of the topographic bench using 3-D linear finite-difference calculations to compute progressive elastic wavefields from explosion sources behind the quarry bench. These 3-D calculations show SH radiation patterns consistent with observations while the SV radiation patterns are not consistent with observations. We find that the radiation patterns from the explosion behind the 3-D bench cannot be modelled by a modified moment tensor. The 3-D effects of the bench are more complicated than the representation by a moment tensor with a single reduced horizontal couple. The 3-D finite-difference synthetics exhibit strong azimuthal asymmetry and polarity reversals in the outgoing P-SV waves (P, S and Rg) radiated behind the bench for Vp/Vs ratios between 2 and 3.
Both mechanisms may contribute to the non-isotropic radiation patterns but the spall mechanism is the simplest physical mechanism that explains the bulk of the observations. Adjustments to the time functions for the horizontal force, the vertical force and the explosion source may further refine the remaining differences between prediction and the observations.
The aerodynamic noise production mechanisms of modern horizontal axis wind turbines are reviewed. An engineering
analysis of the time and frequency scales from three noise sources, leading edge turbulence interaction noise, trailing edge
noise and blade-tower interaction noise is presented. The analysis shows that noise sources are present from low-frequencies
(1-4 Hz) to over 500 Hz for a representative wind turbine. The results of the analysis are used to explain amplitude modulation
observed during noise measurements at a European wind farm. Daytime noise measurements close to a South Australian
wind farm are also presented that show amplitude modulation. The paper concludes with a description of conceptual ideas
for the control of wind turbine noise.
A critical survey of all known published measurement results of infrasound from wind turbines has been made. The survey indicates that wind turbines of contemporary design with an upwind rotor generate very faint infrasound with a level far below the threshold of perception even at a rather short distance. From considerations on propagation and transmission of infrasound it is concluded that infrasound from such upwind turbines can be neglected when evaluating the environment effects of wind turbines. Turbines with downwind rotors produce 10 -30 dB higher infrasound levels, and these may exceed rele-vant assessment criteria for dwellings in the immediate neighbourhood. When longer distances are considered, neither downwind nor upwind turbines are capable of violating assessment criteria for infrasound. This paper considers whether other aspects of the noise than the infrasound can explain the indicated adverse public reactions to large downwind turbines.
A high-pressure gas pipeline exploded near Ghislenghien in Belgium, on 30 July 2004. Seismic energy of this event was recorded at three nearby seis-mometers. The origin time of the explosion was determined by analyzing the Ray-leigh waves and found to be 06h55m272sec coordinated universal time (UTC). Ground truth of the explosion was invoked to assess the accuracy of the location derived from infrasound data recorded over Western Europe. Different infrasonic phases were identified by using array-processing techniques and raytracing through atmospheric models. The analysis showed that the derived location was situated 13 km from the true location. The total area of the uncertainty ellipse, or area to be searched if the source was of unknown origin, was 690 km 2 . After the origin time and location, the yield was calculated by comparing infrasonic stratospheric ampli-tudes with those from a high-explosives dataset and was estimated at 40.9 tons high-explosives equivalent. In summary, this study illustrates the capability of seismic and infrasound data in forensic investigations. Furthermore, the potential of infra-sound as a monitoring or verification technique is addressed.
Expert knowledge suggests that the performance of automated infrasound event association and source location algorithms could be greatly improved by the ability to continually update station travel-time curves to properly account for the hourly, daily, and seasonal changes of the atmospheric state. With the goal of reducing false alarm rates and improving network detection capability we endeavor to develop, validate, and integrate this capability into infrasound processing operations at the International Data Centre of the Comprehensive Nuclear Test-Ban Treaty Organization. Numerous studies have demonstrated that incorporation of hybrid ground-to-space (G2S) enviromental specifications in numerical calculations of infrasound signal travel time and azimuth deviation yields significantly improved results over that of climatological atmospheric specifications, specifically for tropospheric and stratospheric modes. A robust infrastructure currently exists to generate hybrid G2S vector spherical harmonic coefficients, based on existing operational and emperical models on a real-time basis (every 3- to 6-hours) (Drob
et al.,
2003). Thus the next requirement in this endeavor is to refine numerical procedures to calculate infrasound propagation characteristics for robust automatic infrasound arrival identification and network detection, location, and characterization algorithms. We present results from a new code that integrates the local (range-independent) τp ray equations to provide travel time, range, turning point, and azimuth deviation for any location on the globe given a G2S vector spherical harmonic coefficient set. The code employs an accurate numerical technique capable of handling square-root singularities. We investigate the seasonal variability of propagation characteristics over a five-year time series for two different stations within the International Monitoring System with the aim of understanding the capabilities of current working knowledge of the atmosphere and infrasound propagation models. The statistical behaviors or occurrence frequency of various propagation configurations are discussed. Representative examples of some of these propagation configuration states are also shown.
Since the start of the operation of a 30 MW, 17 turbine wind park, residents living 500 m and more from the park have reacted strongly to the noise; residents up to 1900 m distance expressed annoyance. To assess actual sound immission, long term measurements (a total of over 400 night hours in 4 months) have been performed at 400 and 1500 m from the park. In the original sound assessment a fixed relation between wind speed at reference height (10 m) and hub height (98 m) had been used. However, measurements show that the wind speed at hub height at night is up to 2.6 times higher than expected, causing a higher rotational speed of the wind turbines and consequentially up to 15 dB higher sound levels, relative to the same reference wind speed in daytime. Moreover, especially at high rotational speeds the turbines produce a ‘thumping’, impulsive sound, increasing annoyance further. It is concluded that prediction of noise immission at night from (tall) wind turbines is underestimated when measurement data are used (implicitly) assuming a wind profile valid in daytime.
Since regular operation of the DOE/NASA MOD-1 wind turbine began in October 1979 about 10 nearby households have complained of noise from the machine. Development of the NASA-LeRC with turbine sound prediction code began in May 1980 as part of an effort to understand and reduce the noise generated by MOD-1. Tone sound levels predicted with this code are in generally good agreement with measured data taken in the vicinity MOD-1 wind turbine (less than 2 rotor diameters). Comparison in the far field indicates that propagation effects due to terrain and atmospheric conditions may be amplifying the actual sound levels by about 6 dB. Parametric analysis using the code has shown that the predominant contributions to MOD-1 rotor noise are: (1) the velocity deficit in the wake of the support tower; (2) the high rotor speed; and (3) off column operation.
A theory is presented for the discrete-frequency sound radiated by axial-flow fans and compressors. The theory is based on the noise radiation from the fluctuating forces on either a rotor or a stator stage due to interactions with upstream components. The model used is a cascade of point forces, one on each blade, radiating into free air. It appears that, provided that the correct phase relations are retained, this model may be expected to give accurate predictions of farfield noise even for long, hard-wall duct cases. Both over-all power and directionality curves for the sound radiation are presented, and methods are given for calculating the fluctuating forces acting from the wake geometry. Once wake geometry is defined, the theory enables one to perform calculations of the noise observed at any point. Preliminary agreement with experiment is demonstrated. Virtually all significant compressor design parameters can be included in the theory, and therefore it appears that the theory could be used in tradeoff studies to minimize noise at the preliminary design stage of an engine.
Continuous seismic signals of wind turbines (WTs) have been analyzed at 13 sites of the Grafenberg (GRF) array in Germany. The stations of the GRF array have operated continuously for 40 years and comprise the longest available digital broadband array data set. By comparing time spans before and after installation of WTs in the vicinity of the stations, their influence on background noise can be quantified. Here, a strong dependence is shown between local wind speed and the observed effects on noise spectra. Station sites with WTs within distances up to 5 km are exposed to significant disturbance in the background noise; even at distances of 15 km such signals are still visible. The geological setting at GRF with sedimentary layer below all stations seems to favor propagation of these signals. Moreover, we observe different decay patterns for signals below and above 2 Hz, which could be related to the geometry of this layer. Overall, our observations clearly document deteriorating effects of WTs to highly sensitive seismological stations.
The mechanism of noise generation by wind turbines is a subject not yet fully understood. A large number of complex flow phenomena occur, each of which generate sound in particular frequency bands. The purpose of this paper is to provide a brief description of the current state of technology in respect of noise generation from a wind turbine in a complex meteorological atmosphere and reliable methods of noise prediction to sensitive receptors.
The aerodynamic noise production mechanisms of modern horizontal axis wind turbines are reviewed. An engineering analysis of the time and frequency scales from three noise sources, leading edge turbulence interaction noise, trailing edge noise and blade-tower interaction noise is presented. The analysis shows that noise sources are present from low-frequencies (1-4 Hz) to over 500 Hz for a representative wind turbine. The results of the analysis are used to explain amplitude modulation observed during noise measurements at a European wind farm. The paper concludes with a description of conceptual ideas for the control of wind turbine noise.
Infrasound and low-frequency noise below 200 Hz is known to affect the health of human beings. The main purpose of this paper is to experimentally identify the characteristics of acoustic emission of large modern upwind wind turbines with emphasis on infrasound and low-frequency noise. The sound measurement procedures of IEC 61400-11 and ISO 7196 are applied to field test and noise emission from each of 1.5-MW and 660-kW wind turbines utilizing the stall regulation and the pitch control for power regulation is evaluated. The sound spectral density showed that the blade-passing-frequency (BPF) noise is clearly dominant up to 6-7 harmonics, which generally occupy the frequency range of 1-10 Hz, i.e., infrasound. The A-weighted sound pressure levels (SPLs) of the stall control type of wind turbine were found to increase with wind speed in a more correlated way than those of the pitch control type of wind turbine while the G-weighted SPLs of low-frequency noise, including infrasound, were found to show a positive correlation with the wind speed irrespective of the method of power regulation. Potential complaints of local communities about infrasound and low-frequency noise of wind turbines are assessed by comparing the measured data with the existing hearing thresholds and criteria curves. These comparisons show that it is highly possible that low-frequency noise from the 1.5-MW and the 660-kW wind turbines in the frequency range over 30 Hz may lead to psychological complaints by ordinary adults and that infrasound in the frequency range from 5 Hz to 8 Hz could cause complaints due to rattling house fittings such as doors and windows.
Infrasound from a 60-turbine wind farm was found to propagate to distances up to 90 km under nighttime atmospheric conditions. Four infrasound sensor arrays were deployed in central New Mexico in February 2014; three of these arrays captured infrasound from a large wind farm. The arrays were in a linear configuration oriented southeast with 13, 54, 90, and 126 km radial distance and azimuths of 166°, 119°, 113°, and 111° from the 60 1.6-MW turbine Red Mesa Wind Farm (RMWF), Laguna Pueblo, New Mexico, USA.Peaks at a fundamental frequency slightly below 0.9 Hz and its harmonics characterize the spectrum of the detected infrasound. The generation of this signal is linked to the interaction of the blades, flow gradients, and the supporting tower. The production of wind-farm sound, its propagation, and detection at long distances can be related to the characteristics of the atmospheric boundary layer. First, under stable conditions, mostly occurring at night, winds are highly stratified, which enhances the production of thickness sound (TS) and the modulation of other higher-frequency wind turbine sounds. Second, nocturnal atmospheric conditions can create low-altitude waveguides (with altitudes on the order of 100s of meters) allowing long distance propagation. Third, night and early morning hours are characterized by reduced background atmospheric noise that enhances signal detectability. This work describes the characteristics of the infrasound from a quasi-continuous source with the potential for long-range propagation that could be used to monitor the lower part of the atmospheric boundary layer.
The international monitoring system (IMS) is designed to ensure compliance with the comprehensive nuclear-test-ban treaty (CTBT). The infrasound component of the IMS consists of a 60-station network of infrasound monitoring stations distributed uniformly over the face of the globe. This network is designed to reliably detect and locate atmospheric nuclear explosion. IMS infrasound monitoring stations are located in a wide variety of environments ranging from the hot and humid equatorial tropics, to barren wind-swept deserts and the ice-covered wastes of the Polar regions. A large number of signals from a wide variety of sources have been recorded at IMS infrasound stations. Some types of signal are routinely detected at all stations, while other types may be unique to a particular station or region. This chapter provides a thorough survey of the various types of infrasonic signals that have been detected at stations in the global network and a discussion of the potential benefits of the use of infrasonic data from this unique global network in international scientific projects and in geophysical hazard warning systems.
The term infrasound sensor mainly stands for the elements of an infrasound measuring chain, which are located between the wind-generated noise reducer and the recorder. It measures pressure changes over a very large dynamic range and delivers a dynamic signal suited to the associated recorder whose input range is limited. No versatile measuring chain is able to solve the known infrasound low-noise model and to measure largest infrasound all over the infrasound frequency band.
An infrasound sensor can be installed all over the world for a long time under very harsh environmental conditions. Some designs are suited to operate on worldwide networks and are very robust, reliable, and insensitive to environment but sometimes heavy and power consuming. Others are more suited to temporary measurements but are not as reliable or insensitive to environment.
On another hand, sensors performances, reliability, and robustness are very different according to the technique used. The sensor to be used has to be chosen with care according to the specifications to reach and the environmental constraints to undergo.
Objective:
This review examines the literature related to health effects of wind turbines.
Methods:
We reviewed literature related to sound measurements near turbines, epidemiological and experimental studies, and factors associated with annoyance.
Results:
(1) Infrasound sound near wind turbines does not exceed audibility thresholds. (2) Epidemiological studies have shown associations between living near wind turbines and annoyance. (3) Infrasound and low-frequency sound do not present unique health risks. (4) Annoyance seems more strongly related to individual characteristics than noise from turbines.
Discussion:
Further areas of inquiry include enhanced noise characterization, analysis of predicted noise values contrasted with measured levels postinstallation, longitudinal assessments of health pre- and postinstallation, experimental studies in which subjects are "blinded" to the presence or absence of infrasound, and enhanced measurement techniques to evaluate annoyance.
The bane of infrasound signal detection and characterization is the noise generated by wind. In this chapter, we review the physics of wind, current theories on how wind can generate noise, and both mature and developing techniques for reducing wind noise. This subject is not completely documented in peer-reviewed journals, and we have extended our review to include nonpeer-reviewed results that appear to be robust. Specifically, we review Daniels' filters, pipe rosettes, microporous hoses, optical fiber line sensors, distributed sensors, rigid porous media filters, and wind barriers. We discuss the advantages, disadvantages, and potential of each technology. We conclude with a summary of the state of affairs in noise-reduction research and the potential impact of these technologies on future global infrasound monitoring and research efforts.
To monitor compliance with the Comprehensive Nuclear-Test ban Treaty (CTBT), a dedicated International Monitoring System (IMS) is being deployed. Recent global scale observations recorded by this network confirm that its detection capability is highly variable in space and time. Previous studies estimated the radiated source energy from remote observations using empirical yield-scaling relations which account for the along-path stratospheric winds. Although the empirical wind correction reduces the variance in the explosive energy versus pressure relationship, strong variability remains in the yield estimate. Today, numerical modeling techniques provide a basis to better understand the role of different factors describing the source and the atmosphere that influence propagation predictions. In this study, the effects of the source frequency and the stratospheric wind speed are simulated. In order to characterize fine-scale atmospheric structures which are excluded from the current atmospheric specifications, model predictions are further enhanced by the addition of perturbation terms. A theoretical attenuation relation is thus developed from massive numerical simulations using the Parabolic Equation method. Compared with previous studies, our approach provides a more realistic physical description of long-range infrasound propagation. We obtain a new relation combining a near-field and a far-field term, which account for the effects of both geometrical spreading and absorption. In the context of the future verification of the CTBT, the derived attenuation relation quantifies the spatial and temporal variability of the IMS infrasound network performance in higher resolution, and will be helpful for the design and prioritizing maintenance of any arbitrary infrasound monitoring network.
The ability of the International Monitoring System (IMS) infrasound network to detect atmospheric nuclear explosions and other signals of interest is strongly dependent on station-specific ambient noise. This ambient noise includes both incoherent wind noise and real coherent infrasonic waves. Previous ambient infrasound noise models have not distinguished between incoherent and coherent components. We present a first attempt at statistically and systematically characterizing coherent infrasound recorded by the IMS. We perform broadband (0.01-5Hz) array processing with the IMS continuous waveform archive (39 stations from 1 April 2005 to 31 December 2010) using an implementation of the Progressive Multi-Channel Correlation algorithm in log-frequency space. From these results, we estimate multi-year 5th, 50th, and 95th percentiles of the RMS pressure of coherent signals in 15 frequency bands for each station. We compare the resulting coherent infrasound models with raw power spectral density noise models, which inherently include both incoherent and coherent components. Our results indicate that IMS arrays consistently record coherent ambient infrasound across the broad frequency range from 0.01 to 5Hz when wind noise levels permit. The multi-year averaging emphasizes continuous signals such as oceanic microbaroms, as well as persistent transient signals such as repetitive volcanic, surf, thunder, or anthropogenic activity. Systematic characterization of coherent infrasound detection is important for quantifying a station's recording environment, signal-to-noise ratio as a function of frequency and direction, and overall performance, which all influence the detection probability of specific signals of interest. Citation: Matoza, R. S., M., Landes, A., Le Pichon, L., Ceranna, and D., Brown (2013), Coherent ambient infrasound recorded by the International Monitoring System, Geophys. Res. Lett., 40, 429-433, doi:10.1029/2012GL054329.
The ambient infrasound noise environment is characterized for 21 globally distributed infrasound arrays in the frequency band of 0.03 to 7 Hz. Power Spectral Density (PSD) is measured for one site of each array for 21 intervals at each of four times of day from January 2003 through January 2004. The ambient noise at infrasound stations is highly variable by season, time of day and station. Noise spectra for an individual station may vary by four orders of magnitude at any given frequency. Preliminary infrasound noise models are defined, which can be used as baselines for evaluating ambient noise at current and new infrasound stations. Median noise levels in the microbarom band centered on 0.2 Hz vary smoothly in an annual pattern, with most stations observing maximum noise during local winter. Noise amplitudes do not have a normal or log-normal distribution, but rather are skewed to larger amplitudes.
A 59 station infrasound network is being established as part of the verification regime for the Comprehensive Nuclear-Test-Ban Treaty. We present a probabilistic model for predicting the network detection threshold (defined as the chemical explosive yield at which two stations will detect signals with 90% probability). We incorporate the influence of stratospheric wind and detection bandwidth while accounting for estimates of noise amplitude and propagation uncertainties. Inclusion of the stratospheric wind, using a state-of-the-art climatological model, tends to reduce the detection threshold. For 66%/88% of Earth's surface area the detection threshold is lower than that expected for the windless case for all/>=90% of the time. However, preferential downwind detection makes location of explosions with yields close to the threshold more difficult due to the restricted azimuthal coverage. Completing the IMS network is also important: the detection threshold for the 39 station network operating in October 2008 is approximately 45% higher than that predicted for the full 59 station network. Moreover, the absolute detection threshold values are highly sensitive to the frequency band in which the noise is evaluated. A detector simulation comprising overlapping two-octave frequency bands indicates that, when accounting for low noise at frequencies above 0.2 Hz, detection capability at low yields (210 t chemical explosive (>420 t nuclear equivalent yield) will be detected over >=95% of Earth's surface at any time of year (with a 90% probability).
This paper demonstrates the usefulness of continuous infrasound monitoring on regional scales to advance the development of automated source location procedures, as well as provide new insights into quantitative relationships between network detection capabilities and atmospheric specification. To achieve these goals, ground truth events in regions of dense infrasound networks must be considered. The infrasound network of the International Monitoring System (IMS) of the Comprehensive Nuclear-Test Ban Treaty Organization (CTBTO), while currently not fully established, has demonstrated its capability for locating infrasonic sources on a global scale, including meteorites and volcanic eruptions. This paper considers continuous records made by dense networks in Central Europe that are not part of the IMS. It shows the potentiality for future seismo-acoustic monitoring system to be used for discriminating between natural and artificial acoustic sources, and confirms that most of the detected infrasound energy propagates in the ground to stratospheric waveguide.
A large explosion occurred at the Buncefield Oil Depot, United Kingdom,
on the 2005 December 11, as the result of a spillage of 300 tonnes of
petrol. The explosion generated large amplitude infrasonic signals that
were recorded at ranges of up to 1400 km. We present an analysis of data
from across Central Europe using seven infrasound arrays, four of which
recorded a total of 20 infrasonic arrivals, and 49 seismometers, which
recorded 89 air-to-ground coupled arrivals. Exceptionally high amplitude
stratospheric winds occurred at the time of the explosion, propagating
east-southeast with speeds of up to 130 m s-1, generating a
highly efficient duct for infrasound propagation. The Buncefield
explosion can therefore be considered as a benchmark for studying the
influence of different propagation algorithms and atmospheric models on
phase identification, source location and yield estimation. Significant
improvements are made in phase identification using accurate atmospheric
parametrizations and 3-D modelling, where we identify 18 of the 20
observed phases, compared to seasonal atmospheric models and 1-D ray
tracing which only identify seven of the phases. Additionally, various
location scenarios are considered, including cross-bearings and
least-squares traveltime and backazimuth inversion, all of which
estimate locations within 35 km of ground truth. A novel single array
location method using multiple phases works comparably well. Yield
estimates made using the stratospheric arrival amplitudes exhibit one
order of magnitude variability across the network, with a mean estimate
of 51 tonnes high explosive equivalent. Detailed studies, such as this,
are important for developing a successful monitoring regime for
atmospheric or surface events, and for assessing the capability of the
infrasound monitoring component of the Comprehensive Nuclear-Test-Ban
Treaty verification system.
Infrasound from the eruptions of Mt. Etna was detected in The Netherlands at the Deelen Infrasound Array (DIA) during the summer of 2001. Zonal cross winds lead to a deviation in the observed back azimuth. An average azimuthal deviation of 2.6∘ is explained by westward stratospheric winds along the infrasonic trajectories through the atmosphere. The signals have signal-to-noise power ratios lower than 0.6. The detectability of Mt. Etna's infrasound is tested on sub-arrays of the 13 elements of DIA. Doing so, sparse arrays used in the International Monitoring System (IMS) are mimicked. Both small aperture sub-arrays of 800m and large aperture sub-arrays of 1400m are applied. The results are evaluated on the basis of the number of detections made. The number of detections strongly increases with the number of elements. Small aperture sub-arrays perform significantly better than large aperture sub-arrays. With a factor of two reduction in the aperture, two instruments less can be used to obtain the same number of detections. Larger aperture arrays perform less well because of the loss of spatial coherence of the infrasound. The average accuracy of the detections slightly increased, when the number of elements is increased. The result of the use of more elements is that lower signal-to-noise power ratio events could be detected. The trade-off between more detections and the larger average deviation in back azimuth and apparent sound speed causes the slight increase accuracy. The results were comparable to the detections obtained at IMS array IS26 in Germany. It is concluded that successfully applying infrasound as monitoring technique strongly depends on the array configuration. The frequency versus spatial coherence of signals will play a decisive role in detecting low signal-to-noise ratio events.
Atmospheric sound waves in the 0.02-10 Hz region, also known as infrasound, exhibit long-range global propagation characteristics. Measurable infrasound is produced around the globe on a daily basis by a variety of natural and man-made sources. As a result of weak classical attenuation (~0.01 dB km-1 at 0.1 hz), these acoustic signals can propagate thousands of kilometers in tropospheric, stratospheric, and lower thermospheric ducts. To model this propagation accurately, detailed knowledge of the background atmospheric state variables, the global winds and temperature fields from the ground to ~170 km, is required. For infrasound propagation calculations, we have developed a unique atmospheric specification system (G2S) that is capable of providing this information. Using acoustic ray tracing methods and detailed G2S atmospheric specifications, we investigate the major aspects of the spatiotemporal variability of infrasound propagation characteristics.
We present a new method for automatic detection and location of seismic events recorded on a network. It is based on a progressive study of the interstation correlation functions, which leads to a consistent set of time-delays when a seismic phase is present. Three different examples are processed on networks of different sizes.
Observations of atmospheric nuclear tests carried out at the Nevada Test Site from 1951 to 1958 provided data for an empirical investigation of how infrasonic signals are propagated to distances of about 250 km. Those observations and the analysis documented in this report involved signal amplitudes and average velocities and included three classes of signals: stratospheric, thermospheric, and tropospheric/surface. The authors' analysis showed that stratospheric winds have a dominant effect upon stratospheric signal amplitudes. The report outlines a method for normalizing stratospheric signal amplitudes for the effects of upper atmospheric winds and presents equations for predicting or normalizing amplitude and average velocity for the three types of signals.
A listening test has been performed to investigate the relationship between human annoyance and the amplitude modulation of wind turbine noise. To obtain sound samples for the listening test, sound from a 1.5 MW wind turbine in Korea was recorded. The strength of the amplitude modulation of the sound samples was defined in terms of the modulation depth spectrum, which was approximated by assuming that the sound samples are sinusoidally amplitudemodulated. The stimuli for the listening tests were created by reducing the modulation depth spectrum of the sound samples.A total of 30 participants were involved in the listening tests. The results of the listening tests indicate that the equivalent sound level and the amplitude modulation of wind turbine noise both significantly contribute to noise annoyance.
This paper describes new algorithms, not previously available, for predicting atmospheric absorption of sound at high altitudes. A basis for estimating atmospheric absorption up to 160 km is described. The estimated values at altitudes above 90 km must be considered as only approximate due to uncertainties about the composition of the atmosphere above 90 km and simplifying assumptions. At high altitudes, classical and rotational relaxation absorption are dominant, as opposed to absorption by molecular vibrational relaxation that is the principle atmospheric absorption loss mechanism for primary sonic booms propagating downward from a cruising supersonic aircraft. Classical and rotational relaxation absorption varies inversely with atmospheric pressure, thus increasing in magnitude at high altitudes as atmospheric pressure falls. However, classical and rotational losses also relax at the high values of frequency/pressure reached at high altitudes and thus, for audio and infrasonic frequencies, begin to decrease at altitudes in the range of 80–160 km. This paper includes: 1 modifications to the existing algorithms in the ISO/ANSI standards for atmospheric absorption at high altitudes, and 2 algorithms for definition of mean atmospheric conditions, including humidity content at high altitude conditions. Also included are suitable values for the temperature-dependent physical parameters of the atmosphere, viscosity, and the specific heat ratio, involved in defining atmospheric absorption at temperatures found at high altitudes. It has been found that carbon dioxide plays a major role in the relaxation of O 2 and N 2 at high altitudes due to the absence of H 2 O. Molecular relaxation by CO 2 , not covered by the current ANSI or ISO standards, is the dominant source of molecular relaxation absorption at altitudes above 60 km at frequencies of 1 Hz and above 10 km at a frequency of 10 kHz. However, at such high altitudes, classical plus rotational losses dominate reaching maximum values at 80–160 km, depending on frequency. In this regime, vibrational relaxation is less important. More accurate predictions of absorption at altitudes above 90 km would require more sophisticated models for the variation in atmospheric viscosity and specific heat ratio above such altitudes.
Contours of equal loudness and threshold of hearing under binaural free-field conditions for the frequency range 20–15 000 Hz were standardized internationally in 1961. This paper describes an extension of the data in the low-frequency range down to 3·15 Hz, at l levels from threshold to 70 phon. The latter corresponds to nearly 140 dB sound pressure level at the lowest frequency. Direct loudness comparisons were made between tones at intervals of an octave, and the resulting contours were checked by numerical loudness estimation.
Infrasound data were collected using portable arrays in a region of variable terrain elevation to quantify the effects of topography on observed signal amplitude and waveform features at distances less than 25 km from partially contained explosive sources during the Frozen Rock Experiment (FRE) in 2006. Observed infrasound signals varied in amplitude and waveform complexity, indicating propagation effects that are due in part to repeated local maxima and minima in the topography on the scale of the dominant wavelengths of the observed data. Numerical simulations using an empirically derived pressure source function combining published FRE accelerometer data and historical data from Project ESSEX, a time-domain parabolic equation model that accounted for local terrain elevation through terrain-masking, and local meteorological atmospheric profiles were able to explain some but not all of the observed signal features. Specifically, the simulations matched the timing of the observed infrasound signals but underestimated the waveform amplitude observed behind terrain features, suggesting complex scattering and absorption of energy associated with variable topography influences infrasonic energy more than previously observed.
The increasing number and size of wind farms call for more data on human response to wind turbine noise, so that a generalized dose-response relationship can be modeled and possible adverse health effects avoided. This paper reports the results of a 2007 field study in The Netherlands with 725 respondents. A dose-response relationship between calculated A-weighted sound pressure levels and reported perception and annoyance was found. Wind turbine noise was more annoying than transportation noise or industrial noise at comparable levels, possibly due to specific sound properties such as a "swishing" quality, temporal variability, and lack of nighttime abatement. High turbine visibility enhances negative response, and having wind turbines visible from the dwelling significantly increased the risk of annoyance. Annoyance was strongly correlated with a negative attitude toward the visual impact of wind turbines on the landscape. The study further demonstrates that people who benefit economically from wind turbines have a significantly decreased risk of annoyance, despite exposure to similar sound levels. Response to wind turbine noise was similar to that found in Sweden so the dose-response relationship should be generalizable.
Theoretical study of discrete frequency noise generated by turbocompressors and blades
This paper reviews published information on aerodynamically generated noise from large horizontal axis wind turbines operated for electric power generation. Methods are presented for predicting both the discrete frequency rotational noise components and the broadband noise components, and results are compared with measurements. Refraction effects that result in the formation of high-frequency shadow zones in the upwind direction and channeling effects for the low frequencies in the downwind direction are illustrated. Special topics such as distributed source effects in prediction and the role of building dynamics in perception are also included.
The noise radiated by rotor‐stator and stator‐rotor interactions in a duct of negligible length has been analyzed with the aid of the author's general theory. Explicit analytic results for rotating fluctuating source patterns have been found, which reduce to the case of Gutin for the steady propeller case with no axial velocity, and to Garrick and Watkins' results for the propeller problem with axial velocity. Computed results of over‐all power and directionality will be presented for several basic cases, and the practical implication of these theoretical results will be discussed. [This work has been supported by the National Aeronautics and Space Administration under contract.]