Applied Acoustics

Published by Elsevier
Online ISSN: 0003-682X
Publications
Article
The European standard EN 12354-3 describes a calculation model designed to estimate the reduction of outdoor sound by facades of buildings. The accuracy of the prediction method is not specified in the standard. In this paper, the tolerance of the method is studied by comparing the calculation results to the laboratory and field measurement results of different types of facades. For the weighted sound reduction index, the mean and standard deviation of the difference between the calculated and measured values of nineteen facades is 0.3±0.4 dB in laboratory conditions. In field conditions, the mean and standard deviation of the difference between the calculated and measured values of twelve facades is 3.8±3.8 dB. The tolerance is considerably larger compared to the laboratory measurements because the sound insulation of the single building facade elements have been evaluated based on the empirical estimations and not on measurement results.
 
Article
This paper presents the applicability of an in situ technique based on ISO 13472-1 standard for measuring the acoustic absorption coefficient of grass and artificial turf surfaces for normal incidence from a sound source. The in situ method is based on acoustic impulse response measurement of the material surface. A maximum length sequence (MLS) signal is played through a loudspeaker and the acoustic response from the surface is recorded using a single microphone. The fast Hadamard transform and fast Fourier transform based digital signal post-processing algorithm provides the acoustic absorption coefficient of the surface under test. The normal incidence acoustic absorption coefficient of a commercial artificial quash surface of Dow Co. obtained from this method was compared with the results from the ASTM E1050 impedance tube method for the same surface. The acoustic absorption coefficients of a test-site grass surfaces were measured for 30 mm and 100 mm length of grass blades in wet and dry soil conditions. Substantial difference in the acoustic absorption coefficient was observed for a similar grass-like artificial surface used for estimating sound power of commercial garden equipments and lawnmowers. The advantage of the in situ method lies in its ability to measure the normal incident acoustic absorption coefficient of any planar surface as installed or in situ. Additionally a quick testing time of less than a minute with the use of a laptop sound card based inexpensive data acquisition system is the main feature of this robust method.
 
Article
Recent progress in the development of European standards has allowed the in situ testing of roadside noise barriers. CEN/TS 1793-5 describes a test method using maximum length sequences (MLS) for the characterisation of airborne sound insulation. However, many barriers are tested according to a laboratory standard, EN 1793-2, based on measurements carried out in reverberant chambers and in the case of timber barriers with a relatively low airborne sound insulation it is not clear to what extent the results of the two tests compare. The paper describes the results of tests carried out using both methods. Six samples of timber barrier were compared including single-leaf and double-leaf constructions and single-leaf constructions with an absorptive core. Very good agreement was found especially when account was taken of the valid frequency range in each test method. The results open up the possibility of routinely evaluating the performance of timber barriers at the roadside where build quality can be variable and there are concerns that the acoustic performance may not match that obtained under laboratory test conditions where typically the barrier is more carefully constructed.
 
Article
A forecast of changes within the residential areas of constant population density is made in terms of external noise levels () and ‘vehicle travels’ () for cars, trucks and buses. Anticipated increases of and have been estimated by the use of trend line analysis (Figs. 3 and 4). A few hypotheses of and decrease have been considered. Increase of by more than 3 dB may be expected until 1993.
 
Article
This paper describes the methodology through which the UK calculation of road traffic noise (CORTN) has been converted to the algorithms that are able to calculate hourly A-weighted equivalent sound pressure level (LAeq,1h) for the Tehran’s roads. The methodology adopts two different approaches to model calibration and performance test through the holdout validation method on the basis of the database including 52 samples taken from 52 sampling stations located alongside 5 roads of Tehran at distances less than 4 m from the nearside carriageway edge. As to the CORTN manual the distances less than 4 m are considered to be equal to 4 m. In the first approach the model is calibrated through carrying out nonlinear regression parameter estimation using 50% of samples to replace the basic noise level parameters with the new ones that are presumably able to satisfy the objective of the study with an acceptable fitness of the model. In the second approach the model calibration is carried out on the basis of 30 measurements taken from 2 roads. In the next step the other subsets of samples are introduced into the calibrated equations to conduct the performance test. Non parametric goodness of fit tests, i.e. two related samples Wilcoxon and two independent samples Kolmogorov–Smirnov, respectively conducted for the calibration and the performance test steps; indicate satisfactory results for both approaches.
 
Article
The Second International Workshop on Detection and Localization of Marine Mammals Using Passive Acoustics was held in Monaco, 16–18 November 2005, two years after the first workshop on the same topic, held in Dartmouth, NS, Canada in November 2003. This paper is the overview of this workshop.
 
Article
Using the partial-wave series for the acoustic scattering of a high-order Bessel beam (HOBB) of counterpropagating quasi-standing waves of variable half-cone angles, a generalized radiation force expression is obtained. The radiation force function, which is the radiation force per unit cross-sectional surface and unit characteristic energy density, is expressed in terms of the order m of the HOBB, the quasi-standing waves’ amplitudes Φ0 and Φ1, as well as the variable half-cone angles β1 and β2. The features of the theory include the ability to suppress two resonances as well as exploring a broad range of parameters related to the beam shape and mechanical properties of the spherical target.
 
Article
Lam recently introduced an improved diffraction-based method for calculating the insertion loss of a single, finite length, three-dimensional barrier (Applied Acoustics, 42, 29–40). Comparisons of this new method with experimental scale modeling and the wave-based boundary element method showed good agreements over a broad frequency range. This preliminary work of Lam is extended in this paper to include the modeling of two dimensional geometries, the comparison of two- and three-dimensional modeling, the consideration of parallel barriers, modeling the effect of finite ground impedance and the consideration of three-dimensional coherent and incoherent line sources. Results of these developed modeling methods compare well with both boundary element and finite element modeling.
 
Article
In three-dimensional (3D) freehand ultrasound (US), reconstructing a set of B-scans into a regular voxel array is the key procedure for consequent visualization and analysis. This paper presents a new adaptive interpolation algorithm for computing the voxel array to suppress speckle noises and enhance contrast. The local statistics of homogeneous regions including mean and variance were measured and the ratio of variance to mean was used as homogeneity criteria. For the computation of each voxel, the interpolation method was adaptively determined with respect to its local statistics. If the neighbouring pixels of a voxel satisfied the homogeneity criterion, its value was computed with an arithmetic mean filter. Otherwise, the voxel was probably locating in an inhomogeneous region and an adaptive distance-weighted (ADW) interpolation method was employed to compute its value. A resolution phantom and a subject’s forearm were reconstructed using the proposed algorithm and two other well-known methods – conventional distance-weighted (DW) and voxel nearest neighbourhood (VNN) interpolations. The comparison results demonstrated that the adaptive interpolation algorithm was able to suppress speckles, preserve edges and enhance contrast effectively for the volume reconstruction.
 
Article
This paper proposes an algorithm for whale monitoring by passive acoustic using four short-spaced hydrophones. It is successfully tested on the NEutrino Mediterranean Observatory – Ocean Noise Detection Experiment (NEMO ONDE) underwater acoustic platform. In past years, interest in marine mammals has increased, leading to the development of passive acoustic methods of localization which permits to study whales’ behavior in deep water (several hundreds of meters) without interfering with the animals. In this paper, we propose a robust Angle of Arrival (AoA) tracking algorithm, which provides an estimation of elevation and azimuth angles in spite of the short distance between hydrophones, which does not allow direct 3D localization. Moreover we show that, in some cases, the time-delay between direct signal and its reflection on the sea surface allows the range estimation, and thus the exact localization of the whale. The classic AoA estimation is performed with a non-linear regression, and it is compared with a more sophisticated algorithm, the Rao-Blackwell Monte Carlo Data Association. We demonstrate that the second method is very robust to the presence of clutter. According to the Pavi 2009 workshop challenge, our algorithm is the only one performing relevant tracks of whale.
 
Article
This paper provides a novel echo-robust and real-time passive underwater acoustic method to track whales using five omni-directional widely-spaced bottom-mounted hydrophones. The interest in online marine mammal monitoring for behavioral studies of endangered species has increased. Practically, these systems are required to be real-time and robust to underwater echoes, which dramatically affect tracking results. To meet these demands, a real-time tracking algorithm was developed with simple but efficient echo cancellation. The processing is all done in the time–domain. After signal decimation, rough time delays of arrival (TDOAs) are calculated, selected and filtered, before precise TDOAs are re-estimated on the original signals. The complete algorithm is tested on real data from the Naval Undersea Warfare Center and the Atlantic Undersea Test & Evaluation Center—Bahamas. The attractive advantage of the method is that it runs two times faster than real-time, but generates similar tracks to other state-of-the-art methods.
 
Article
Acoustic imaging and characterisation of buried objects (and in particular archaeological materials) in shallow-water (<5 m) is often unsuccessful owing to problems related to vessel-induced bubble turbulence and the restricted acoustic geometry of the system. A 2D surveying method that tackles these problems has been tested on the known wreck of the Grace Dieu (1418), Henry V’s flagship, currently buried within the inter-tidal sediments of the Hamble River. The wooden hull is recognisable in the seismic sections as a high amplitude anomaly underlain by an acoustic blanking zone. Close survey line spacing (ca. 1 m) allowed the construction of time slices, identifying the ovate plan of the hull. High, predominantly negative, reflection coefficients suggest this anomaly corresponds to degraded oak timbers buried within the sediment. Combining the data enabled the construction of a (pseudo)-3D image, revealing the dimensions and shape of the hull remains for the first time.
 
Principle of decomposition of a line source into series of point sources
shows a comparison of cap efficiency obtained for an incoherent line source (located 12 m behind the barrier) and for a summation of point sources every 10 and 20 degrees (from  i = 0 to 80°, plus 85° and 88°).
Test-wall measurement set up at CSTB-Grenoble with loudspeaker and microphone. Without (top view) and with cap (back view).
Comparison of the octave band T-shaped cap efficiency, in dB: measurements (solid lines) and MICADO calculations (dashed lines)
Article
Road barrier diffracting caps have shown a renewed interest for several years since they give the opportunity of increasing the barrier efficiency without changing its overall height. First investigations on the efficiency of road barrier caps calculated with a boundary element method (BEM) have shown that the efficiency obtained with coherent line sources is underestimated compared to that with incoherent line sources, more representative of road traffic noise. The present work deals with the characterisation of the real performance of a T-shaped absorbing cap with road traffic noise conditions. Two different approaches are compared: on one hand calculations with the help of a BEM program able to achieve 2D and 2D simulations are made; on the other hand outdoor measurements on a test-wall using a maximum length sequence technique are carried out. The goal in the two approaches is to isolate the top edge diffracted sound field in order to determine an extrinsic value of octave band efficiency of the cap for many source–receiver pairs. These results integrated in a ray tracing prediction method enable the integration of air absorption along each ray path and give the real efficiency of such a device in the case of complex and realistic configurations for barriers of finite length.
 
Article
This study analyzes and investigates the impact of traffic noise on the high-rise building and surrounding area by the side of a new motorway that links Bangkok to the new Suwannaphum International Airport and Pattaya. A traffic noise simulation model in 3D form is applied on a GIS system. Visualized noise levels are formulated in vectored contours for noise mapping on all surfaces of the building and surrounding ground in a 3D platform. Noise impact is then investigated based on this 3D noise mapping in LAeq,1 h noise contours. The investigation shows that there is a high traffic noise impact on the foreground and front façade of the building, rendering this area unsuitable for residential purposes. The ground area by the sides of the building and the building side panels receive a lower noise impact. Most of these areas are still not acceptable for residential use; however, all of the side panels and most of the ground area by the sides of the building can be used for commercial and business purposes. The back yard and back panel, together with the rooftop, have the lowest traffic noise impact. They are the safest places for use as residential areas, except for a small strip along the front edge of the rooftop. From this study, residential areas that are sensitive to noise impact must be located far away from the front façade and side panels of a building. It is also shows that the building height is not an effective means of reducing motorway noise on the upper part of the building.
 
Article
A simple passive acoustic monitoring (PAM) setup was used to localize and track beluga whales underwater in three dimensions (3D) in a fjord. In June 2009, beluga clicks were recorded from a cabled hydrophone array in a regularly frequented habitat in Eastern Canada. Beluga click energy was concentrated in the 30–50 kHz frequency band. The click trains detected on several hydrophones were localized from their time difference of arrivals. Cluster analysis linked localizations into tracks based on criteria of spatial and temporal proximity. At close ranges from the array, the localized click-train series allowed three-dimensional tracking of a beluga during its dive. Clicks within a train spanned a large range of durations, inter-click intervals, source levels and bandwidths. Buzzes sometimes terminated the trains. Repeated click packets were frequent. All click characteristics are consistent with oblique observations from the beam axis, and ordered variation of the source pattern during a train, likely resulting from a scan of angles from the beam axis, was observed before click trains indicated focusing of the echolocation clicks in one direction. The click-train series is interpreted as echolocation chasing for preys during a foraging dive. Results show that a simple PAM system can be configured to passively and effectively 3D track wild belugas and small odontocetes in their regularly frequented habitat.
 
Article
The boundary element method (BEM) is used to evaluate the acoustic scattering of a three-dimensional (3D) sound source by an infinitely long rigid barrier in the vicinity of tall buildings. The barrier is assumed to be non-absorbing and the buildings are modeled as an infinite barrier. The calculations are performed in the frequency domain and time signatures are obtained by means of inverse Fourier transforms. The 3D solution is obtained by means of Fourier transform in the direction in which the geometry does not vary. This requires solving a series of 2D problems with different spatial wavenumbers, kz. The wavenumber transform in discrete form is obtained by considering an infinite number of virtual point sources equally spaced along the z axis. Complex frequencies are used to minimize the influence of these neighboring fictitious sources. Different geometric models, with barriers of varying sizes, are used. The reduction of sound pressure in the vicinity of the buildings is evaluated and the creation of shadow zones by the barriers is analyzed and compared with results provided by a simplified method.
 
Article
High speed switching of current in gradient coils within high magnetic field strength magnetic resonance imaging (MRI) scanners results in high acoustic sound pressure levels (SPL) in and around these machines. Many studies have already been conducted to characterize the sound field in and around MRIs and various methods have been investigated to attenuate the noise generated. In the work presented here a computational vibro-acoustic model was developed based on an iteratively modified and validated finite element (FE) model to characterize the acoustic noise properties of the gradient coil. The simulation results from the computational model were verified through experimental noise measurement for the gradient coil insert in a 4 T MRI scanner by using swept sinusoidal time waveform inputs. Comparisons show that the computational model predicts the noise characteristic properties extremely accurately. There are three dominant frequency bands where the SPL is much higher than those at other frequencies. The SPL in the horizontal direction is much higher than that in the vertical direction due to the excitation to the horizontally placed X coil. The SPL to the inner surface of the coil is higher than far from the inner surface, which proves that the acoustic noise is radiated from the inner surface and primarily caused by the normal vibration of the inner surface. Further verification was conducted by using two types of trapezoidal sequence inputs usually used, which is to simulate real scanning sequences for small animals. Again the accuracy of the developed model is verified. The validated acoustic computational model could be used as an effective method to predict the noise that would be produced by a coil in the design stage. Modification of the structural design or the excitation pulse could be performed to reduce the acoustic noise when the gradient coil is in scanning.
 
Article
In this study, we introduce outdoor sound simulation that is fully compliant with ISO 9613 yet with some complementary methods that enhance its applicability; for example, calculation of sound attenuation due to undulating terrain in octave bands, geometric divergence in the near-field of the source, and short-term wind effects. Using the method, we have carried out highway traffic noise prediction and measurement for 12 sites with representative road shapes and structures. In the prediction, the sound power level for a road segment was estimated by the method suggested in ASJ Model-1998 with experimental corrections to the overall noise level and spectrum. Comparing results between predicted and measured noise levels show good correspondence at direct, diffracted and reflected sound fields within 30m from the center of the near side lane.
 
Article
Three types of vegetation can be discerned with regard to the acoustic climate investigated in 4 different plantations, 2 plant communities, and a grass field. In the first type of vegetation, i.e. beech and ashtree forests, the excess attenuation was at least 10 dB/100 m with the receiver at the same height as the source, 1·2 m, and at least 5 dB/100 m with the receiver at 3·9 m and even more in most -octave bands studied. In the second type of vegetation, i.e. the mixed poplar forest and the Stellario carpinetum, a so-called ‘sound window’ could be detected around 2 kHz and the ground effect was more extended towards the high frequency range compared with the first type of vegetation. In the third type of vegetation consisting of evergreen sprucefir, best excess attenuation was found in the closed forest: at least 10 dB/100 m with the receiver at 1·2 m, and 7 dB/100 m with the receiver at 3·9 m; in the belts the excess attenuation was at least 7 dB/100 m and 4 dB/100 m respectively. Best attenuation was, therefore, found in the closed forest, and not in the belts. It was concluded that the sound attenuating capacity of planted vegetations can be used to abate noise pollution in town and landscape planning if the plantations are at least 12 m wide. To obtain the best effect the rows of trees have to be planted perpendicular to the direction of the sound field.
 
Article
Parts 1 and 2 of this comprehensive synopsis of a relatively new and still developing field of noise control engineering have discussed the various physical mechanisms as well as the hydrodynamic and hydroacoustic scaling laws of the many different sound sources inherent in, mainly, sanitary installations. Disturbance characteristics were described and international regulations for the limiting noise immission levels compared. Following the philosophy of a waterborne sound emission test procedure, Part 3 suggests a new measuring standard for the usually much more complex structure-borne sound generation processes in buildings. It concludes with a display of various types of sound abatement principles and their respective noise reduction potential when properly applied in practice.
 
Article
The presence of turbulence in the atmosphere affects the interaction between an acoustic wave and the ground surface. The noise attenuation by the ground in the presence of atmospheric turbulence is smaller than in non-turbulent atmosphere.A simple engineering model of noise propagation above a flat ground surface, for stationary and moving point sources, has been proposed. The model takes into account the air absorption and ground effect in the presence of turbulence.As well as parameters for type of ground and air absorption, the model introduces two adjustable parameters which must be deduced from in situ measurements at two ranges or two heights. The model’s free parameters have been obtained as a function of the resultant sound speed gradient on the basis of the field measurements performed for a stationary noise source. Also, using field data for a vehicle moving at steady speeds up to 100 km/h, the model has been verified for a moving point source.
 
Article
A laser pistonphone for the absolute calibration of microphones at low frequencies has been developed at UME. The motion of an electro-dynamically driven piston in a small close cavity produces a sound pressure. Accurate measurement of the piston displacement, by self-mixing interferometry, enables this sound pressure to be calculated, and consequently the pressure sensitivity of a microphone, exposed to this sound pressure, to be determined. Absolute calibrations of type LS1P and WS1P microphones have been carried out with an uncertainty of less than 0.15 dB. The performance of the laser pistonphone has been validated by comparing the measured microphone sensitivities with those obtained by the closed coupler reciprocity method.
 
Article
The purpose of this paper is to analyze the acoustic performance of resonators with porous material inside the expansion chamber. Sound wave propagation in the porous media is an extremely complicated phenomenon. In the present study the dynamic density and bulk modulus of air in the porous media are used to describe the viscous effects and the thermal effects in an air-saturated rigid frame porous material. With this assumption the absorbent material in the expansion chamber can be equivalent to a fluid with complex sound speed and wave number. Then the coupled equations for sound wave propagation in air (in straight tube) and the equivalent fluid (in expansion chamber) are solved by the numerical decoupling method. Finally, the effects of porous material on the performance of a resonator are discussed. ©
 
Article
A modification of the diffusion model’s boundary condition, based on the Eyring absorption coefficient, to account for high walls absorption is proposed. Numerical comparisons are carried out for three geometrical configurations (a proportionate room, a corridor and a flat enclosure). Comparisons with the statistical theory and a ray-tracing software show that the modified boundary condition increases the accuracy of the diffusion model in term of reverberation time in all the simulated configurations. An experimental comparison in the case of a non-uniformly absorbent room (a reverberation chamber covered with patches of glass wool) is also carried out. The modified-diffusion model results match well with the ray-tracing ones. Both models are in agreement with the experimental data for most of third octave bands (discrepancy close to or below 10%). However, some discrepancies up to 40% can also be observed in a few octave bands, probably due to experimental considerations and to the modal behaviour of the room at low frequencies.
 
Article
This paper introduces a three-part report describing research into the use of Millington absorption coefficients in the computer modelling of sound fields in enclosed spaces with absorbent room surfaces. The historical background to the prediction of reverberation time is presented together with three types of computer models used in the investigation. In part one, the computer models are described, the Millington reverberation time formula is validated, Millington absorption coefficients are derived and the sound field in a concert hall is predicted. This enables the accuracy of the three types of computer models to be compared and the effect of applying different absorption coefficients to be studied. Part two of the report consists of an extensive investigation into the prediction of reverberation time in multiple configurations of an experimental room with absorbent material partially covering the room surfaces. This determined the accuracy of reverberation time formulae and the computer models using both standard and Millington absorption coefficients under controlled conditions. The final part contributes a verification of the accuracy of the predictions using Millington absorption coefficients in a factory space with a barrier installed, and a refined diffraction model based on a ray-tracing model.
 
Article
The optimal design of a dynamic absorber can be classified into time domain optimization and frequency domain optimization. In this paper, four optimum design methods for a dynamic absorber are compared when they are applied to a single-degree-of-freedom system with primary damping. Furthermore, they can be used on other complex systems, including continuous systems. These four optimum methods are (1) the equal peak method, (2) the minimal variance method, (3) the energy method, and (4) the area ratio method. The design objective of these methods is to seek the optimal tuning and damping ratios of the dynamic absorber, whether in the time domain or in the frequency domain.
 
Article
A new theoretical model for predicting the sound absorption of perforated absorber systems is presented. The system with a perforated panel consists of an arbitrary combination of porous and air layers backed by a hard wall. This model includes the effect of the diffraction phenomenon caused by impedance discontinuities of the boundary surface, which has been disregarded in the past studies. Numerical results for extensive materials are compared with existing experimental data, which are in fairly good agreement over the whole frequency range of practical use.
 
Article
The increase of the sound transmission loss of an aircraft panel caused by dynamic vibration absorbers has been analysed theoretically. The statistical energy analysis approach is employed where the indirect coupling loss factors are considered in order to overcome the normal deficiency of this method at low frequency. The function of the dynamic vibration absorbers is reflected in the equivalent loss factor of the panel and therefore in the indirect coupling loss factors among the subsystems. A modal analysis method is used to obtain the equivalent loss factor of the panel. Experiments are carried out and good agreement with theoretical results is obtained. It is shown that a dynamic vibration absorber can be efficient in increasing the sound transmission loss of the panel in the frequencies of application.
 
Article
A new type of acoustic liner developed for broadband noise reduction in flow ducts is considered in this paper. It combines passive absorbent properties of a porous layer and active control at its rear face. The complete design procedure of this hybrid passive/active liner is developed here. The passive part is first considered with the determination of a suitable porous material and the cut-off frequency separating the active low frequency regime from the passive high frequency one. The control system is then presented: a digital adaptive feedback control is performed independently cell by cell, allowing an easy subsequent increase of the liner surface. The entire optimization process has been successfully applied to a laboratory flow duct: both predictions and measurements show the interest of the hybrid liner to reduce the noise radiation.
 
Article
Panel-type sound absorbers are commonly used to absorb low-frequency sounds. Recently, a new type of panel/membrane absorbers has been proposed as a next-generation sound absorber free from environmental problems. On the other hand, it is known that placing a honeycomb structure behind a porous layer can improve sound absorption performance and a similar effect can be obtained for microperforated-panel absorbers. Herein, the sound absorption characteristics of a panel sound absorber with a honeycomb in its back cavity are theoretically analyzed. The numerical results are used to discuss the variations in the sound absorption characteristics due to the honeycomb as well as the mechanism for sound absorption.
 
Article
Porous or fibrous materials are used in a great variety of applications to absorb acoustic energy at medium and high frequencies above approximately 200 Hz. There are, however, cases where their open and rough surface entails certain disadvantages with respect to hygiene and cleaning requirements. For medium and low frequencies, which would also require a relatively large absorber thickness and weight, there is a need for an alternative absorber, the acoustically active components of which are formed exclusively by, ideally, even and smooth membranes. The sound absorber presented herein is of the reactive or resonant type with several different modes of vibration excitable in a complex system of rather thin, though comparatively stiff metal or plastic membranes. The absorption of the vibrational energy as stimulated by the sound waves impinging on the new type of acoustic lining or splitters is brought about solely by frictional forces in bounded shear layers formed in between specially shaped membranes moving against each other and relative to the air volumes adjacent to and between them. When suitably adjusted to the particular noise spectrum, these combined vibrational and damping mechanisms enable the construction of a new generation of sound absorber which no longer requires additional porous material to be incorporated in order to make it effective in a broad band of medium and low frequencies. The honeycomb structure, which forms the solid frame for the acoustic absorber, also makes it a relatively light and stable construction element for both room acoustic and industrial applications, where contamination of the absorber by dust in the air or pollution of the environment by abrasive deposits from the damping material is to be avoided under all circumstances.
 
Article
The membrane absorber is a fibre-free absorber which consists of vibrating metal membranes on a lightweight metal honeycomb structure. An advantage of this absorber is that the smooth cover-membrane seals the component against humidity and solid particles. In some cases the cover-membrane still needs to be frequently cleaned, and a flowing water-film can be used for this purpose. In this research the effects of such a water-film on the absorption characteristics of the membrane absorber have been experimentally investigated in a reverberation room. The results show that with a water-film the absorption coefficient of a membrane absorber is generally decreased, and the frequency of maximum absorption becomes slightly lower. It is noted, however, that when the water-film is relatively thin, say 1 mm, with which a membrane absorber can be effectively cleaned, its effect on sound absorption is negligible. Generally speaking, a vertically flowing water-film has less effect on the absorption of membrane absorbers than a horizontal water-film. Measurements also show that the noise level caused by a flowing water-film is acceptable in practice.
 
Article
In multivariate systems, when it comes to identifying actual operating conditions ranges, or optimal settings, the use of constrained optimization is often required. Among the different tools for the engineer to perform such optimization, designed experiments offer accurate performances. In this paper, the optimization process of “electroacoustic absorbers” is investigated by means of response surface methodology. A multivariate linear model is established by a series of designed experiments in order to analyze the modification of electroacoustic absorber performances due to the variation of several constitutive parameters (such as the moving mass of the loudspeaker, the enclosure volume, the filling density of mineral fiber within the enclosure, and the electrical load value to which the loudspeaker is connected), that influence their whole absorbing mechanisms. A simple case study is then provided to illustrate the capabilities of the developed optimization procedure, from which general conclusions on such design methodology, as well as on electroacoustic absorbers sensitivity, are drawn.
 
Article
The paper examines the feasibility of using transparent micro-perforated absorbers (MPA) in a window system to allow noise attenuation whilst at the same time maintaining high levels of comfort ventilation and daylighting. The underlying theory for micro-perforated panels and membranes and the application in silencers is presented. Experiments have been performed between a semi-anechoic and a reverberant chamber using a standard window mock-up, and the effectiveness of MPA has been demonstrated. With a constant air backing, MPA are more effective with a wider ventilation path. With an air flow of up to 2 m/s the performance of the MPA remains unchanged. Current results are based on readily available materials and relatively simple configurations, but the theoretical analysis suggests possibilities for increasing the noise reduction and widening the frequency range by using more strategically designed materials and configurations.
 
Article
The present work is related to acoustic in situ free-field measurements of sound absorption in porous materials, such as cellular plastic foams, glass–wool or recycled felt materials. The emphasis is given towards fine metrology of absorption in view of potential industrial applications. A powerful ultrasonic array working at 40 kHz is used. It enables to measure absorption acoustical data down to 100 Hz due to the exploitation of the non-linear ultrasonic demodulation phenomenon in air. Fine measurements of acoustic absorption are compared to numerical predictions based on the “equivalent-fluid” model (when the squeleton frame is motionless), and to some measurements performed on a Brüel and Kjaer impedance tube. Dispersion curves are also measured and compared to the numerical predictions for some automotive felt materials which are compressed at various ratios. Data obtained with a dedicated portable instrument are also discussed for the same type of materials and configurations.
 
Article
Four types of fibrous material samples made of polyester fibres of various cross-sections, i.e. circle, hollow, flat and triangle, have been prepared. The surface impedance and the non-acoustical parameters of them have been measured.The effect of the variation in the cross-sections on the acoustical properties and the non-acoustical parameters of the polyester fibre samples have been examined. The effects of the ‘circle’ and the ‘hollow’ samples are almost the same. However, for the ‘flat’ and the ‘triangle’ samples, the effects different from the ‘circle’ sample are found in the measured flow resistivity and the two characteristic lengths. This paper also discusses the equivalent diameters of the materials made of non-circular cross-sections.
 
Article
In the past few years the intensity method has become a practical tool for measurements in building acoustics. However, some complications may arise when using this method to determine the transmission loss of building elements.One of these complications concerns the possibility of a measurement deviation in highly reactive environments, caused by absorption at the receiving side of the partition wall.In this paper an expression for this measurement deviation is derived. For a measurement set-up with rockwool attached to the partition wall, the calculated and the measured deviation proved to be in fair agreement.
 
Article
Nowadays architects commonly use the ‘coupled space concept’; examples are mezzanines, half-open office spaces and exhibition rooms. There is a need to predict acoustical quantities for this category of spaces, since half-open spaces may be a cause of noise annoyance. The transmission of sound between coupled spaces depends on design decisions like position, shape and dimensions of the surfaces and on the reflection characteristics. This paper deals with some problems related to the application of absorbing surfaces in coupled rooms, especially when they are modelled in a ray-tracing program. Absorption coefficients from meausurements in reverberation chambers may exceed 1.0 and they do not bear any information about angle dependent behavior, so an extra conversion must be made into input values for the ray-tracing model. Therefore ray-tracing calculations have been performed in a computer model of a reverberation chamber. From a comparison study between measurements and calculations in three coupled rooms it is found that the accuracy is good, provided that the sound reflections on the walls are introduced as angle dependent. Care should be taken in choosing a diffusion factor of flat surfaces.
 
Article
Atmospheric absorption is important for outdoor sound propagation. Weather changes over the day and the year, and this alters the atmospheric absorption. Thirty years of meteorological measurements from six stations in Sweden were used to determine the conditions for atmospheric absorption. The size of the monthly and daily variances was frequency-dependent. The most accurate information about atmospheric absorption climate is achieved if hourly values over 30 years are used. Errors using 1-year hourly data or only the mean values of temperature and humidity for the calculations were determined. Atmospheric absorptions for various parts of the world were estimated. An increase in the accuracy of calculations of outdoor sound levels can be made by taking the local climate into consideration.
 
Article
Coconut is one of the most important harvests in Malaysia. Industrial prepared coir fiber is obtained from coconut husk combined with latex and other additives to enhance its structural characteristics. Unfortunately, such inevitable process diminishes the acoustical features of material. Previous studies on industrial coir fiber and fiber–air gap layers showed that low frequency absorptions needed improvements. Therefore perforated plate (PP) was added to the multilayer structure to further enhance the sound absorption in this area. Analyses were accomplished through three PP modeling approaches (Allard, Beranek and Ver, Atalla and Sgard) and Allard Transfer Function (TF) method. Experiments were conducted in impedance tube to support the analytical results. Outcomes showed that Allard TF method was generally closer to measurement values and implemented for additional analyses. Two possible conditions of putting PP in front of fiber layer or between fiber–air gap layers were investigated. Both arrangements were suitable to enhance the sound absorption. Although, when PP was backed by coir fiber and air gap, porosity of the plate had great influence in adjusting the amount of low frequency absorption. Result derived that PP might improve the low frequency absorption of coir fiber but at the same time the medium frequency absorption was reduced. This effect was noticed previously in coir fiber–air gap structures while the air gap thickness increased. The advantage of using PP was that it assisted in greatly reducing the air gap thickness under the same acoustical performance. Hence it is an efficient tool to reduce the thickness of acoustic isolators in practical purposes.
 
Article
This paper investigates the potential of active absorbers for reducing low-frequency noise transmission through an enclosure. Active absorbers are intended to obtain a purely real prescribed impedance at the front face of a porous layer. This is achieved by an active control system which cancels the acoustic pressure at the rear face. The test bench was a simplified enclosure: a rigid-wall cavity coupled to a baffled elastic plate. The modeling of the system was based on an analytical modal approach. The purpose of this simulation was first to calculate the optimal impedance, providing maximal reduction in radiated power, and then to define a sub-optimal strategy for actual absorber production. Two 3-cell configurations were implemented on the test bench. Active control used a multichannel feedforward algorithm. In line with prediction, the absorbers provided a 5.5 dB overall reduction while covering only 2% of the cavity surface.
 
Article
The addition of sound absorbing materials such as fibreglass in the cavities of lightweight walls to improve sound insulation is a well-known technique. In this paper, the potential of another fibrous material, namely polyester fibre, for such applications is studied. The effect of material parameters such as mass/area, fibre type, crimpness, and binding fibre content on the sound transmission loss of a lightweight wall system has been studied. The flow resistivities of polyester fibre materials were measured to determine the suitability of an empirical prediction equation developed for fibreglass to polyester fibres. A new prediction equation relating the flow resistivity to the number of fibres/volume in the material is presented. It is also shown that the design charts for estimating sound absorption coefficients of fibrous absorbers using the flow resistivity parameter can be used to predict the absorption coefficients of polyester fibres.
 
Article
The sound absorption of an industrial waste, developed during the processing of tea leaves has been investigated. Three different layers of tea-leaf-fibre waste materials with and without backing provided by a single layer of woven textile cloth were tested for their sound absorption properties. The experimental data indicate that a 1 cm thick tea-leaf-fibre waste material with backing, provides sound absorption which is almost equivalent to that provided by six layers of woven textile cloth. Twenty millimeters thick layers of rigidly backed tea-leaf-fibres and non-woven fibre materials exhibit almost equivalent sound absorption in the frequency range between 500 and 3200 Hz.
 
Article
This study investigated the absorption characteristics of materials in a multi-purpose hall using computer models, 1:10 scale model and actual hall measurements of Gimhae Arts Hall (GAH), in order to predict and evaluate the acoustical characteristics. The elements of this scale model, such as reflecting walls, seats, audience, and absorption banners, were made with materials selected according to their absorption coefficients, measured in a 1:10 scale model reverberation chamber. After the real hall was completed, in situ acoustical measurements were conducted in the GAH and compared with those of the scale model hall. Comparison of these measurements showed that the delay time of the major reflections in the scale model hall was similar to that of the real hall. However, the reverberation time especially at low frequencies showed a difference between the scale model hall and the real hall measurements. The results of computer simulations for both scale model and actual hall showed that the absorption of seats and audience, the structural detail of the reflecting walls with different thickness and air spaces, and the duct facilities in the open-type ceiling are the major differences. It was confirmed that there are more complicated absorption characteristics in the scale model design of a multi-purpose hall than a concert hall.
 
Analysed irregular shaped room in a form of two connected rectangular subrooms. Asterisk shows position of point sound source.
Shapes of eigenfunctions W n for mode numbers: (a) n = 1, (b) n = 10, (c) n = 20, (d) n = 30, (e) n = 40, (f) n = 50. Black and white colors denote minimal and maximal values of W n , respectively.
Comparison between reverberation time T and modal reverberation times T mn (vertical lines) for absorbing material located on: (a) floor and ceiling, (b) lateral walls, (c) all room walls.
Article
In this paper, a modal analysis was used to describe a reverberation phenomenon in a room of complex shape. A theoretical model was limited to low sound frequencies, when eigenmodes are lightly damped, thus they may be approximated by uncoupled normal acoustic modes of a hard-walled room. A utility of this method was demonstrated in a numerical example where the enclosure in a form of two coupled rooms was considered. A reverberation time was evaluated from a time decay of spatial root mean square pressure, the overall measure of room pressure. The results of calculations, performed for three different distributions of absorbing materials on room walls, showed how various location of the material can effect a dependence of the reverberation time on a frequency of sound source.
 
Article
Prediction of noise levels at shielded positions in urban areas is more difficult than on exposed positions. At shielded positions, the predictions method must include multiple reflections, and many sources must be taken into account. Using numerical methods that solve the wave equation is possible, but very computationally heavy. Here two methods have been used, a very simplified ray model and a statistical model. The results show that concentrating the traffic and introducing absorption onto building façades will give lower levels at shielded positions.
 
Article
An in situ measurement method is proposed for obtaining the normal surface impedance and absorption coefficient of porous materials using two microphones located close to the material without a specific sound source such as a loudspeaker. Ambient environmental noise that does not excite distinct modes in the sound field is employed as the sound source. Measurements of the normal surface impedance of glass wool and rockwool have been made using this method in various sound fields. The repeatability and wide applicability of the method are demonstrated by comparing results of measurements in one room with different noise conditions and in three other environments (corridor, cafeteria and terrace). The assumed diffuse nature of the sound field on the material is validated by using absorption characteristics obtained experimentally at oblique incidence. This method allows simple and efficient in situ measurements of absorption characteristics of materials in a diffuse field.
 
Article
An improved method is proposed for revealing surface anomalies in structures by in situ relative measurement of acoustic energy absorption. Characterization and mapping of particular structures, such as plasters of antique mural paintings are taken into account as a possible application. In situ measurements of the acoustic energy absorption are carried out, employing a diagnostic technique based on the Cepstrum algorithm. Tests on samples with artificially prepared specimens, simulating surface anomalies, are described and a collection of acoustic images is presented. Properties of the specimens and features of the experimental technique are discussed.
 
Article
The absorption coefficient of acoustic materials can be measured either in the frequency or the time domain. At normal incidence, a sample of the material is fitted within an impedance tube and the absorption coefficient is calculated in the frequency domain from the measurement of the transfer function between two microphones [ISO 10534-2. Acoustics – determination of sound absorption coefficient and impedance in impedance tubes – Part 2: transfer function method. ISO, Geneva, Switzerland; 1996]. When the acoustic material must be characterized at oblique incidence or in situ (noise barriers, for instance) the absorption coefficient is calculated from measurements of the loudspeaker–microphone impulse response in the time domain, both in free field and in front of the sample [CEN/TS 1793-5. Road traffic noise reduction devices – test method for determining the acoustic performance – Part 5: intrinsic characteristics – in situ values of sound reflection and airborne sound insulation. CEN, Brussels, Belgium; 2003, ISO 13472-1. Acoustic measurement of sound absorption properties of road surfaces in situ – Part I: extended surface method. ISO, Geneva, Switzerland; 2002]. Since the absorption is an intrinsic property of the acoustic material, its measurement in either domain must provide the same result. However, this has not been formally demonstrated yet. The aim of this paper is to carry out a comparison between the absorption coefficient predicted by the impedance model of a Microperforated Insertion Unit and the absorption coefficient predicted from a simulated reflection trace taken into account the finite length of the time window.
 
Article
The influence of pews on the acoustical characteristics of churches may be significant because they occupy large areas in rooms lacking absorbing surfaces. However, the information about this particular category of seating is sparse and, sometimes, contradictory. Different types of pews, differing in materials and construction, were analysed in a reverberant chamber by means of Bradley’s method, measuring the absorption coefficients of blocks with different perimeter-to-area ratios. A substantial dependence on the latter parameter was found, allowing prediction of absorption coefficients as a function of actual block dimensions. The presence of upholstered kneelers showed improved absorption properties which were also replicated on other pew types by adding strips of polyester fibre. On-site measurements in three churches were used to validate the method by means of direct application of Sabine’s formula and by means of virtual acoustic modelling. The first only provided satisfactory results in rooms complying with Sabine’s assumptions, while the second also gave good results in the other church with markedly non-uniform absorption. Finally, the application to computer models of absorption coefficients measured in the chamber was discussed.
 
Top-cited authors
Jian Kang
  • University College London
Kimihiro Sakagami
  • Kobe University
Lotfi Saidi
  • Higher School of Sciences and Technology (ESST) http://www.essths.rnu.tn/public/
Victor V. Krylov
  • Loughborough University
Jaouher Ben Ali
  • Ensit Tunsia and FEMTO-ST France