Noise Control Engineering Journal

Online ISSN: 0736-2501
Publications
Article
The carbon nanotube (CNT) thermophone has been explored as a novel loudspeaker. Potential advantages of this technology in the audio industry include ultra-lightweight, low production cost, compact size, and independence from rare-earth materials. In this paper, progress toward a practical CNT loudspeaker is presented. Large, high quality CNT thin-film assemblies are designed and built. Design guidance for these types of assemblies is provided. Maximum sound output level, total harmonic distortion, and power efficiency tests are performed. A maximum source level of 111 dBA at 1 m is achieved at 2 kHz with the new sources. The main hurdle to this technology remains power efficiency. Several paths forward are discussed as this technology continues to advance to a position where it may be able to compete with current state-of-the-art, moving-coil loudspeakers.
 
First segmentation step (identical for AzB and Doc.29): combination of track segments in the ground plane with vertical aircraft profile segments. Result is a spatial trajectory represented by flight-path segments. is the length co-ordinate along the flight track.
Article
The European Environmental Noise Directive END requires harmonized calculation methods for the major environmental noise sources. Currently two potential candidates are discussed for the modeling of aircraft noise: the 3rd edition of ECAC Doc.29 was published in 2005 describing a recommended method for the calculation of aircraft noise around civil airports. The German AzB model published in 2008 was developed as a tool for the enforcement of the new German Act for Protection against Aircraft Noise that came into force in 2007. This paper gives a comparative overview on both methods, the underlying acoustic and aircraft performance models and their usability for different fields of application.
 
Article
The use of Helmholtz resonators to increase the sidewall transmission loss (TL) in aircraft cabin sidewalls is evaluated. Development, construction, and test of an aircraft cabin acoustic enclosure, built in support of the Propfan Test Assessment (PTA) program, is described. Laboratory and flight test results are discussed. Resonators (448) were located between the enclosure trim panels and the fuselage shell. In addition, 152 resonators were placed between the enclosure and aircraft floors. The 600 resonators were each tuned to a propfan fundamental blade passage frequency (235 Hz). After flight testing on the PTA aircraft, noise reduction (NR) tests were performed with the enclosure in the Kelly Johnson Research and Development Center Acoustics Laboratory. Broadband and tonal excitations were used in the laboratory. Tonal excitation simulated the propfan flight test excitation. The resonators increase the NR of the cabin walls around the resonance frequency of the resonator array. Increases in NR of up to 11 dB were measured. The effects of flanking, sidewall absorption, cabin absorption, resonator loading of trim panels, and panel vibrations are presented. Resonator and sidewall panel design and test are discussed.
 
Article
A measurement technique is developed in order to obtain the sound transmission loss of an aircraft fuselage which obviates the need for the two-room transmission suite. The sound transmission paths were determined in tests on a light aircraft fuselage using a two-microphone acoustic intensity method for measuring the acoustic intensity transmitted to the interior when the fuselage was exposed to an external random incidence sound-field. The intensity transmitted through different sections of the fuselage can be estimated accurately using this new technique. Results of these tests show that the plexiglass window is the major transmission path in the high frequency range. In addition, the transmission losses through a single and a double layer window were predicted theoretically by using the Statistical Energy Analysis Model. Very good agreement is found between the predictions and the measurements.
 
Article
The oil pan of large diesel engine trucks is identified as a significant contributor to their external noise radiation. This undesired broadband noise is caused by the oil pan's structural vibration and cannot be treated effectively by passive measures, especially in the low frequency range up to 500 Hz. In order to address this challenge, an Active Structural Acoustic Control (ASAC) system was to be realized within the European Union (EU) project �Intelligent Materials for Active Noise Reduction� for the oil pan of a Volvo MD13 truck engine. The technological demonstration was conducted in a laboratory test stand and employs piezoceramic patch actuators to alter the vibrations in a broadband noise reduction manner. The following paper focuses on the tasks of deriving actuator placement and estimating respective broadband sound power minimization capability. First, the oil pan's structural response to typical force excitation at its mounting points was analyzed through finite element simulation, and an actuator placement was derived. The estimation of its sound power minimization capability was then established based on computed structural frequency response functions and numerically derived radiation modes. Finally, the actuator layout was transferred to a serial production oil pan and adjusted by experimental observations
 
Article
An algorithm for generating appropriate velocity boundary conditions for an acoustic boundary element analysis from the kinematics of an operating propeller is presented. It constitutes the initial phase of Integrating sophisticated rotorcraft models into a conventional boundary element analysis. Currently, the pressure field is computed by a linear approximation. An initial validation of the developed process was performed by comparing numerical results to test data for the external acoustic pressure on the surface of a tilt-rotor aircraft for one flight condition.
 
Article
The National Aeronautics and Space Administration (NASA), Langley Research Center (LaRC), sponsored the Acoustics Facility at the United States Department of Transportation's John A. Volpe National Transportation Systems Center (Volpe Center) and the Massachusetts Institute of Technology (MIT) to conduct a noise measurement study at Logan International Airport in Boston, Massachusetts, during the summer of 1999 to examine the applicability of currently available mathematical models of lateral attenuation. Analysis of the data collected revealed that lateral attenuation is a function of aircraft geometry. Lateral attenuation for aircraft with tailmounted engines was found to agree with the published literature, as well as that included in existing aircraft noise models. Lateral attenuation for aircraft with wing-mounted engines was found to be less than documented in the literature. This lower lateral attenuation for aircraft with wing-mounted engines results in a general under-prediction of side-line noise in the existing noise models.
 
Conference Paper
This paper presents results of a research project on active control of the blade passage frequency tone of an axial fan. The secondary sound field is generated by aeroacoustic sources, which are produced by actively controlling the flow around the impeller blade tips. Both amplitude and phase can e controlled in such a way that a destructive superposition with the primary sound field is possible. The flow distortions can be achieved by using different actuators; results using steady and unsteadyjets of compressed air and piezo-electric actuators are presented. (C) Institute of Noise Control Engineering.
 
Article
It is pointed out that interior noise levels of propeller driven aircraft are substantially higher than levels measured for other types of CTOL aircraft. Reduction of interior noise of such aircraft requires a knowledge of the relative importance of the acoustic and structural noise transmission paths. Noise entering the aircraft interior via an acoustic path is the noise radiated by an external noise source (propellers, exhaust noise), which propagates through the acoustic medium (air) and is then transmitted through the aircraft fuselage. This type of incoming noise is referred to as airborne noise. Noise entering the aircraft interior via a structural path is the noise which has its source in the vibrational energy which has been transmitted through the structure from a remote vibrational energy source (engines, wind flutter). This type of incoming noise is referred to as structureborne noise. A new method for separating airborne and structureborne noise is presented. It is based on two-microphone cross spectral acoustic intensity measurements.
 
Article
An experimental technique was used to measure structural intensity through an aircraft fuselage with an excitation load applied near one of the wing attachment locations. The fuselage was relatively large, requiring several measurement locations to analyze the intensity flow through the whole of the structure. For the measurement of structural intensity, the use of a transducer array was necessary at every location of interest. A trade-off was therefore required between the number of measurement transducers, the mounting of these transducers, and the accuracy of the measurements. Using four accelerometers mounted on a bakelite platform, structural intensity vectors were measured at locations distributed throughout the fuselage. The results of these measurements, together with a discussion on the suitability of the approach for measuring structural intensity on a real structure, are presented in this paper.
 
Article
Aircraft noise disturbs sleep and impairs recuperation. Integrative noise measures like LDN or Lnight are commonly used to implicitly describe and limit nocturnal aircraft noise effects. However, noise exposures may differ in their sleep disturbing potential but nevertheless calculate to the same LDNor Lnight. Therefore, it has recently become more popular to use exposure-response relationships for single noise events to explicitly predict the degree of sleep fragmentation for a whole night with multiple noise events. These models have been criticized for not accounting for dependence between consecutive noise events, for disregarding the placement of noise events within the night, for disregarding other important acoustic parameters beside maximum sound pressure level (SPL) or single event level (SEL), and for not considering interindividual differences in noise sensitivity. In this analysis, a Basic Model containing maximum SPL as the only explanatory variable and an Extended Model containing elapsed sleep time, time between maxima, and noise duration as additional variables are compared according to bias and precision in predicting the number of noise induced awakenings in single nights. Random subject effect logistic regression based on field study data, where the reaction to 16279 aircraft noise events was monitored in 64 subjects and 479 subject nights, was used for both models. The results indicate that a variable should only be included in the prediction model if (a) the variable has a relevant impact on sleep, (b) information on the distribution of the variable in the target population is available, and (c) the distribution of the variable in the target population differs relevantly from the distribution in the population that was used to generate the prediction model.
 
Article
This report describes a study of the noticeability and annoyance of intruding noises to test participants who were engaged in a distracting foreground task. Ten test participants read material of their own choosing while seated individually in front of a loudspeaker in an anechoic chamber. One of three specially constructed masking noise environments with limited dynamic range was heard at all times. A laboratory computer produced sounds of aircraft and ground vehicles as heard at varying distances at unpredictable intervals and carefully controlled levels. Test participants were instructed to click a computer mouse at any time that a noise distinct from the background noise environment came to their attention, and then to indicate their degree of annoyance with the noise that they had noticed. The results confirmed that both the noticeability of noise intrusions and their annoyance were closely related to their audibility.
 
Article
Conventional models of sound propagation between porous walls (Scott, 1946) are adapted in order to calculate design curves for the lined circular and annular-duct silencers used in HVAC systems. The derivation of the governing equations is outlined, and results for two typical cases are presented graphically. Good agreement with published experimental data is demonstrated.
 
Article
The present study focussed on awakenings caused by nocturnal noises emitted from aircraft, road and rail vehicles with maximum levels ranging from 40 to 76 dBA. A laboratory study with 24 participants (12 male, 12 female, 19-28 years) was performed with polysomnographic recordings during 13 nights (including a preceding habituation night). Multivariable random subject effect logistic regression models containing acoustical, situational and individual parameters were used to determine the probability of event-related awakenings for each traffic mode. Awakening probability increased significantly with maximum sound pressure level (SPL), slope of rise (dB/s), noise duration and the noise-free interval between noise events. Gender, noise sensitivity and age did not influence awakening probability significantly (the latter only in a combined model). Awakening probability increased with elapsed time after sleep onset, and was significantly lower during slow wave sleep compared to S2 sleep, but not during REM sleep. After adjusting for differences in study design (acoustical macrostructure), awakening probability decreased in the order rail, road and air traffic noise, but only rail and air traffic noise differed significantly (p=0.002). After further adjusting for slope of rise and noise duration (acoustical microstructure), differences between traffic modes decreased, but rail and air traffic noise still differed significantly (p=0.044). Acoustical properties other than slope of rise and noise duration may account for the residual difference. The results of this study suggest that the reduction of maximum SPLs, rise slopes, and traffic volume during the second part of the night might reduce the number of noise-induced awakenings.
 
Article
Modern high speed propeller (advanced turboprop) aircraft are expected to operate on 50 to 60 percent less fuel than the 1980 vintage turbofan fleet while at the same time matching the flight speed and performance of those aircraft. Counterrotation turboprop engines offer additional fuel savings by means of upstream propeller swirl recovery. This paper presents acoustic sideline results for a full-scale counterrotation turboprop engine at cruise conditions. The engine was installed on a Boeing 727 aircraft in place of the right-side turbofan engine. Acoustic data were taken from an instrumented Learjet chase plane. Sideline acoustic results are presented for 0.50 and 0.72 Mach cruise conditions. A scale model of the engine propeller was tested in a wind tunnel at 0.72 Mach cruise conditions. The model data were adjusted to flight acquisition conditions and were in general agreement with the flight results.
 
Article
Sound power levels were measured for an axial-flow fan and a centrifugal fan, both having 600-mm-diam impellers. One-third-octave-band sound power levels were measured by microphones at essentially free-field locations and in the test ducts for two installation configurations: (1) an open inlet and an anechoic outlet duct and (2) an open outlet and an anechoic inlet duct. Two parallelepiped 'rectangular box' measurement surfaces and a hemispherical measurement surface were used for free-field tests. The requirement in ISO 3744 that the environmental correction be not more than 2 dB was not satisfied at some frequencies in the frequency range of interest. On average, the smallest environmental corrections were observed for the smallest measurement surface. Differences, as great as 4 dB, between the free-field sound power levels were largest at frequencies for which the environmental correction was also large. For plane sound waves, in-duct sound power levels were greater than the free-field sound power levels. At frequencies where higher-order-mode sounds propagated through the test ducts, sound power levels determined by the in-duct method were less than those determined by the free-field method. This paper discusses reasons for the differences between the sound power levels by the two methods.
 
Article
The human perception of a nuisance level of noise was quantified in tests and attempts were made to define criteria for acceptable sound levels from wind turbines. Comparisons were made between the sound necessary to cause building vibration, which occurred near the Mod-1 wind turbine, and human perception thresholds for building noise and building vibration. Thresholds were measured for both broadband and impulsive noise, with the finding that noise in the 500-2000 Hz region, and impulses with a 1 Hz fundamental, were most noticeable. Curves were developed for matching a receiver location with expected acoustic output from a machine to determine if the sound levels were offensive. In any case, further data from operating machines are required before definitive criteria can be established.
 
Article
Various papers concerning computers for noise control are presented. The general topics considered include: numerical methods in noise control, computer-aided testing, office equipment and environment, computer-aided design and modal analysis, noise control solutions, damping and brake squeal, and the application of CAD to ducts and mufflers. Other general topics addressed include: signal processing and diagnostics, plant and community noise, personal computers and spreadsheets, intensity measurements, and propellers/fans/cutters.
 
Article
The propagation of sound outdoors, from a source to a listener, is shown to be affected by the ground's acoustic impedance, interference, diffraction, refraction and scattering wave propagation phenomena, and such meteorological conditions as wind and temperature, and their fluctuations. It is established that short and long range propagation are affected by different mechanisms, and that community noise prediction schemes and test procedures for vehicles and other noise generators have economic and social implications which have become more significant in recent years and call for commensurate prediction technique improvements.
 
Article
The conventional passive tuned vibration absorber (TVA) can substantially reduce the vibration of a primary system only when it is tuned to the excitation frequency. To overcome this narrow-band problem, some adaptive tuned vibration absorbers (ATVAs) were developed in recent years. ATVAs possess natural frequencies that can usually be adjusted by altering absorber stiffness. Another problem of this device is that using the TVA can result in the resonance of a primary system since a new composite system is formed when a TVA is attached to a primary system. In this paper, a novel control strategy based on variable mass (VM) ATVA is proposed to reduce the resonance of the primary system. Unlike previous designs, the natural frequency of the VM ATVA can be adjusted by changing its mass, and the control plan can be implemented in the same manner. A numerical simulation and some experiments were conducted to test the performance of the new control plan, and the results show that the suggested plan can achieve a resonance reduction of approximately 17.6 dB in the simulation and about 11.3 dB in the experimental system. This finding implies that the VM absorber that uses the proposed control plan can effectively suppress the vibration of a primary system over a wide frequency band and effectively reduce its resonance. Moreover, the new control strategy is actually a generalizable method that can be used on other types of ATVA.
 
Article
A microperforated panel (MPP) absorber is known as one of the most promising alternatives of the next-generation sound absorbers. However, MPPs are usually made of thin limp materials and need to be reinforced by a supporting element. The authors proposed to use a honeycomb attached behind an MPP for this purpose, and have shown that the honeycomb in the back cavity of an MPP absorber is not only useful to stiffen the MPP but improves its sound absorption performance, particularly at low frequencies. In the authors″ previous studies an electro-acoustical circuit model was used to analyse its sound absorption characteristics, however, the model inevitably includes an approximation, and more exact theory needs to be established for better prediction of its characteristics. In this paper, the absorber is analysed with the wave theory based on Helmholtz-Kirchhoff integral equations. First, the formalism based on the Helmholtz-Kirchhoff integral is presented. Next, an experiment is made to validate the theory. Finally, a parametric study is made to discuss the effect of the parameters of the sound absorbing system on its sound absorption characteristics.
 
Article
Low frequency noise measurements in reverberation and free-field rooms require special attention as the sound field in this frequency range may be dominated by only a few modes (eigenfrequencies). This paper discusses how reverberation rooms may be adequately damped to smooth the spatial sound field and to increase measurement reproducibility and repeatability without violating standards. Absorber modules which are only 250 mm thick are described and their operating principles are discussed. The modules are used as linings for test cells and wind tunnels to provide free-field conditions down to 50 Hz for 1/3-octave band measurements. Their unprecedented acoustic effectiveness is demonstrated by fundamental investigations in well-conditioned test rooms and final verification in a number of rigorously silenced and anechoic facilities completed in the research and development centers of 5 different automobile manufacturers in Europe and the United States. Since the novel acoustic materials and modules also provide several advantages concerning spaciousness, cleanability and durability their practicability is demonstrated in a few typical examples of approximately 50 completed measuring environments.
 
Article
Micro-perforated panel (MPP) absorber with multi-size holes is a new type of broad band sound absorber which is thin, durable, clean and environmentally friendly. These advantages make it especially suitable for applications in the narrow space. However, so far it has not been widely used partially because of its design complexity. The absorption performance of a single MPP absorber with uniform-size perforations depends on four structure parameters: the hole diameter, the perforation ratio, the panel thickness and the cavity depth. It is possible to find an appropriate combination of these parameters that provides sound absorption in one or two octave bands within the frequency range of interest for noise control applications. However, the design of an N-size MPP absorber depends on 2 + 2N parameters which greatly increases design complexity. To overcome this problem, multi-population genetic algorithm (MPGA) is proposed in this study for the optimization design of a multi-size MPP absorber. A theoretical model of multi-size MPP absorbers is developed firstly and then its validity is verified by experimental data. Thereafter, a program for optimizing for wide band noise over a specified frequency range has been created and run. Results indicate that MPGA provides a fast and efficient approach for designing multisize MPP absorbers.
 
Article
Methods and results of utilizing time-delay spectrometry (TDS) for determining absorption characteristics of glass fiber materials at normal and oblique incidence angles are presented. With recent advances in TDS, free-field measurements can now be made over a continuous frequency range with improved dynamic range, and newly available microprocessor-based analyzers provide a portable, quick analysis system with on-line data reduction qualities. Results are given of using TDS to determine noise level reductions of aluminum panels in unmounted and mounted configurations. Good correlation with theory is seen.
 
Article
The role of acoustic guide signals is to lead visually impaired persons to their destination points. However, little scientific evidence indicates that acoustical guide signals that are currently used in Japan can play that role. This study was undertaken to evaluate the sound-localization performance of typical acoustical guide signals under noisy conditions and in a reverberant sound field. Results of listening tests suggest that 1) effects of noise on sound localization can be presented as a function of the signal-to-noise ratio, 2) the initial delay time and reverberation energy are more important than the reverberation time, and 3) a temporal pattern of a guide signal is an important factor for sound localization in reverberant sound fields.
 
Article
The importance of flow noise is increasing with the increasing popularity of quiet electric vehicles. In general, flow noise covers a wide frequency range and usually the high frequency flow noise is important for the vehicle quality because of the increased sensitivity of human perception to high frequency noise. For predicting high frequency noise, statistical energy analysis (SEA) is often used in the prediction of acoustic insulation and absorption. Some of SEA modeling for flow noise transmission has been reported in the literature. However, most of the SEA modeling needs CFD results which require huge computation time. In vehicle design, sometimes the time required for improvements does not permit time for development of CFD models. In such situations, easy and fast diagnostic method like energy transmission contribution analysis is required. In this paper, the mechanism of the flow noise transmission is reviewed especially in the case of low speed flow of around 80 km/h. The acoustic wavenumber component of fluctuating surface pressure is dominant source of flow noise transmission for low speed flow. Therefore, the acoustic wavenumber component identification using conventional acoustic SEA model and the wind tunnel test results is presented. The identified acoustic wavenumber component is validated by comparing with the acoustic wavenumber component identified from the wavenumber-frequency spectrum to that computed using a CFD model.
 
Article
In this paper, a coupled numerical method of the edge-based smoothed finite element (ES-FEM) with the fast multipole BEM (FM-BEM) is proposed to analyze structural acoustic problems. The vibrating structure is modeled using the so-called ES-FEM-DSG3 method, where the 3-node linear triangle plate elements based on the Reissner-Mindlin plate theory with the discrete shear gap (DSG) technique for overcoming the shear locking are applied. The edge-based gradient smoothing operations are applied to “soften” the “overly-stiff” behavior in the standard FEM, which significantly reduces the inherent numerical dispersion error. The normal velocities on the surface of the structure are imposed as boundary conditions for the acoustic domain which is modeled using the FM-BEM for both the interior and exterior acoustic domains. Comparing with the conventional BEM, the matrix vector multiplication and the memory requirement in the FM-BEM are reduced dramatically. The coupled ES-FEM/ FM-BEM method takes the advantages of both ES-FEM and FM-BEM, which can avoid drawbacks of the “overly-stiff” behavior in FEM and computational inefficiency in the conventional BEM. Two numerical examples are presented to verify and demonstrate the effectiveness of the combined method: one academic problem for studying in detail the accuracy and efficiency of the present method, and one application to a practical vehicle noise simulation.
 
Article
Problems related to fan sound power measurements are discussed, with special emphasis on estimation of the sound power radiated into a duct system. The influence of internal fan impedance and duct impedance on the sound produced by the fan is assessed. An axial flow fan is considered for both the mathematical modeling and the experimental investigation. Attention is directed to the development and verification of a dipole model for fan sound generation, refinements of the dipole source model, and application of the dipole model to formulation of fan sound test standards. Implications of the dipole model for the acoustic design of fan duct systems is also discussed.
 
Article
Barrier experiments were conducted in a low speed wind tunnel in which the flow simulates the mean and fluctuating components of the turbulent velocity of the atmospheric boundary layer over uniform terrain. The tests consisted of 1:32 scale model experiments of sound propagating over grassland, both with and without an acoustic barrier, and also with and without the presence of turbulent winds. Results show two effects of the wind on the performance of barriers: (1) the barrier attenuation is increased for upwind propagation and decreased for downwind propagation, and (2) the fluctuations in the measured levels of the wind reduction were of the same order as the mean value.
 
Article
The turbulent boundary layer excitation is one of the main sources of aircraft interior noise over a large frequency range while the diffuse sound field excitation is often considered as an approximation for noise analysis and noise control evaluation. This paper presents the study of add-on damping patches efficiency under both excitations. The structure of interest here is a simply supported thin plate partially covered with a damping treatment. Numerical results are carried out using an original methodology based on the Minimization of Error on a Sample of Observation Points (MESOP). The transmission loss index is assessed in the frequency range from 50 Hz to 7 kHz. This investigation points out different behaviors: a damping effect is observed only in the critical frequency region under diffuse sound field excitation whereas the transmission loss index is increased by damping over the whole frequency band under turbulent boundary layer excitation. The excitation field appears to be a key factor. The excitation effect on the damping treatment characterization is then observed for two patch sizes. The transmission loss under aerodynamic excitation with varying patch sizes is finally investigated in a context of damping treatments optimization. (C) 2010 Institute of Noise Control Engineering.
 
Article
This paper studies a simultaneous optimization model for low-frequency coupled structural-acoustic problems, including three optimization levels, i.e., size, shape and topology variables. Adjoint subspace iteration method is adopted to deal with eigenfrequency problems of acoustic-structural coupled systems. Numerical methods of sensitivity analysis and design optimization are reviewed. Detailed formats of sensitivity analysis are given for different design variables. The direct acoustical characteristic, interior average sound pressure level (SPL) is chosen as the design performance for the simultaneous optimization. This work is an extension of the general-purpose FE analysis and optimization software—JIFEX developed by Dalian University of Technology. Numerical examples and engineering application are given in this paper to demonstrate the effectiveness of the methods and program.
 
Article
Micro-perforated panel (MPP) absorbers, well known as a basis for the next generation of sound absorbing materials, are now being widely used in noise control engineering. In order to design the structural parameters of MPP absorbers according to the actual demand, a straightforward method to predict the absorption performance of such absorbers is needed. However, traditional predicting methods, such as equivalent electric-acoustic circuit method, the transfer matrix method, modal analysis method and so on, are based on analytical solution. The use of these methods not only requires development and application of special techniques, but also is not suitable for MPPs with irregular-shaped holes, such as tapered holes which can be used to improve the sound absorption performance of a thick MPP absorber. In order to overcome the problem, a numerical procedure based on finite element method (FEM) is developed to obtain the specific acoustic impedance of an MPP. Using this method, the acoustic performance of MPPs with tapered holes as well as the effect of various parameters on their normal incidence absorption performance is numerically investigated and the findings are useful to guide the structural design.
 
Article
The public address (PA) is a part of the passenger information system in an underground subway station. A sample station named Xingzhong Road on metro line 9 in Shanghai is considered in this paper. A ray-tracing model for acoustical simulation of the PA system is established. Verification is performed by comparing the simulated and the measured sound pressure levels (SPLs), early decay time (EDT), reverberation times (RTs) and clarity (C50). Several sound samples are simulated by changing the ceiling absorption materials of the subway station model. Jury tests of these sounds are conducted by using the paired-comparison method (PCM). We found that people prefer a shorter RT, which demands higher sound absorption materials. The relationship between the RTs and the averaged absorption ratios of the ceiling materials suggests that the RT around 1 s is the best in the Xingzhong Road station, and the values of the objective parameters, such as the SPL, EDT, speech transmission index (STI) and echo speech, are acceptable to PA system. The method presented in this paper can be regarded as a useful tool for acoustical optimization design of the subway station PA system.
 
Article
A method is evaluated for reducing both the harmonic sound and the random noise generated by centrifugal fans by placing sound absorbing lining into the volute of the housing. Experimental results presented in this paper show that the A-weighted sound pressure level in the fan outlet duct can be lowered by up to 12 dB, while the reduction measured in the inlet duct is insignificant. No attempt was made to optimize the wall impedance of the liner. The aerodynamic fan performance is affected only little by the modification of the casing because its geometry remains the same.
 
Article
Airflow through a corrugated tubing can generate loud tonal sounds. The production of these tones, sometimes called as whistling, in a corrugated tubing is an interesting phenomenon because the presence of corrugations in a straight tube transforms the tube into an aerodynamic sound source that may produce significant tones and resonance responses. Corrugations add flexibility to both metallic and non-metallic tubings, making them useful in many engineering, industrial and household applications. Even though corrugations can be used to produce pleasant musical tones in a children's toy called “Magic Whistle,” it can cause noise problems in vacuum cleaners and also results in severe structural failure in “singing risers” on off-shore gas platforms, HVAC ducts and air-conditioning system pipe lines used in aircraft and automotive applications. In this study, interaction of flow with corrugations is modeled with CFD-LES using ANSYS-FLUENT. The predicted whistling frequencies are then compared with experimental values. It is observed that there is a good agreement between experiment, theoretical and numerical analysis. The work also presents a parametric study of eight types of corrugated tubes with different cavity length, width and pitch on their acoustical behavior. The study indicates that decreasing cavity depth and increasing number of corrugations per given length would result in reduced noise level.
 
Article
The effect of inlet guide vanes (IGVs) on the acoustic and performance characteristics of an automotive centrifugal compressor is studied on a steady-flow turbocharger experimental facility. Broadband noise accompanying flow separation occurs as the flow rate of the compressor is reduced and the incidence angle of the flow relative to the leading edge of the inducer blades increases. The addition of IGVs upstream of the inducer imparts a tangential (swirl) velocity component in the same direction of impeller rotation, which improves the incidence angle particularly at low to mid-flow rates. In the present study, experimental data is compared for three compressor inlet geometries, including a no-swirl baseline along with two different IGV configurations. IGVs were shown to slightly improve the surge line at the highest rotational speed considered in this study, while compromising the maximum flow rate at all rotational speeds. In the high-flow range, IGVs are observed to increase compressor inlet noise levels over a wide frequency range. At the more common low to mid-flow range, however, broadband whoosh noise is reduced with the addition of IGVs in the 5-12 kHz band.
 
Aluminum extrusion with multiple sound treatment materials.
The STL of a door with different slits.
nstallation of the specimen when viewed from the receiving room.
Comparison of theory and measurement for the STL of 2 mm and 3 mm flat aluminum themselves and their combined structures with damping.
Article
Frame structures in urban rail trains have been manufactured with extruded aluminum panels. This study proposes prediction methods of the sound transmission loss (STL) of the aluminum extrusion panels and their combined structures with different acoustical treatment materials. The main contributions of this paper are (1) the establishment of STL calculation models of aluminum extrusions and their combined structures with damping or with fillings of sound-absorption materials; (2) proposed selection principles of damping material at aluminum extrusions surface and (3) specification of maximum size of the gap between sidewall doors or gangway less than 3 mm. Analysis method can be utilized to predict the sound insulation performance of the aluminum extrusion panels in the early design stage or suggest improvements.
 
Article
The sound exposure level, SEL, is the measure of noise coming from a single aircraft operation. All SEL measurements, performed in the vicinity of an airport, are aggregated into a few categories of normal distribution. Each SEL category is characterized by the category mean, variance, and the occurrence probability. This study shows how to calculate the overall mean and overall standard deviation of all possible values of SEL from the known occurrence probabilities and the measured category means and variances. The airport noise can be assessed in terms of SEL, but also in terms of the time average sound level, LpAeq,T and the day-evening-night level, Lden. The occurrence probabilities, category means, and variances of SEL are used to estimate the overall mean and overall standard variation of LAeqT and Lden.
 
Article
This paper reports the results of an initial experiment to evaluate the additional annoyance caused by varying amounts of low-frequency rumble sounds from heating, ventilating, and air conditioning (HVAC) systems. HVAC noises were simulated with various levels of low-frequency sound and varying amounts of amplitude modulation of the low-frequency components. Subjects listened to the test sounds over headphones and adjusted the level of the test sounds to be equally annoying as a fixed neural reference sound. The results indicated that annoyance is influenced by both the level and the amplitude modulation of the simulated HVAC rumble sounds. A procedure that incorporates these two factors is suggested for predicting the additional annoyance of HVAC sounds containing rumble.
 
Article
Factors which must be given careful consideration in anechoic chamber design are reviewed. There should not be reflections of sound from its walls, ceiling or floor in an ideal chamber. The one design parameter which has the most obvious bearing on cost, and is therefore perhaps the most significant factor, is the chamber size. In order to be effective, the chamber should follow some basic guidelines for minimum dimensions. These are determined from the size and type of sound source, the required measurement distances, the lowest frequency of interest, the wedge depth, and the wall thickness.
 
Article
In an application for a U.S. patent, Lueg (1934) introduced the concept of active attenuation of sound by using an artificially produced acoustic wave mixed with the unwanted sound so that the waves were 180 deg out of phase and destructive interference resulted by design. Unfortunately, the electronic technology of the 1930s was not sufficiently advanced to meet the control requirements of active noise attenuation systems. In connection with advances in the technology, an 'electronic sound absorber' was described by Olson and May (1953), and Conover (1956) discussed the active control of transformer noise. Further advances in technology which began prior to the mid-1960s, and which are continuing, made the active control of noise much more attractive than it had ever been before. Attention is given to developments regarding electronics and geometries, the three-dimensional problem of transformer noise, and the attenuation of sound in ducts.
 
Article
The impedance tube method is widely used for measuring sound absorption (or reflection) coefficients of acoustic materials as a function of frequency. However, the sound absorption coefficients obtained using the impedance tube method may have some variations due to the dimensions (limits) of an impedance tube, sample preparation and sample mounting. This paper assesses the performance of the two-microphone impedance tube method as a function of frequency for different tube dimensions and materials and presents suggestions for increasing the reliability and repeatability of impedance tube measurements. First, after summarizing a systematic way for measuring acoustic transfer functions, sound absorption coefficients of a variety of materials ranging from conventional absorbing acoustic materials to samples with thin films are measured using two tubes with different tube diameter and microphone spacing. Uncertainty of sound absorption coefficients for various materials is discussed, and the frequency limits of impedance tubes are assessed. Then, a method for minimizing uncertainty due to sample mounting is proposed and the main findings are discussed.
 
Article
Open cavities subjected to a grazing air flow produce high level aeroacoustic noise consisting of intense discrete and broadband components. The discrete components are generated when the air flow induces acoustic waves that drive the resonance modes of the cavity. The acoustic resonances in the open cavity can be reduced by noise control techniques. One passive control technique consists of lining the inner walls of the cavity with absorbing material. While porous materials provide wideband absorption, they are discouraged in presence of air flow. Microperforated panels (MPP) have been proposed as the next generation absorbers that can be used in presence of air flow. Furthermore, their usually high manufacturing cost can be drastically reduced by an infiltration technique. This paper reports the reduction of resonance tones when an open cavity is lined with MPPs fabricated by infiltration.
 
Article
Transmission loss (TL) of a simple expansion chamber (SEC) consists of periodic domes with sharp troughs. This limits practical application of the SEC in the variable-speed automobile exhaust systems. Three-fourths of the troughs of the SEC can be lifted by appropriate tuning of the extended inlet/outlet lengths. However, such mufflers suffer from high back pressure and generation of aerodynamic noise due to free shear layers at the area discontinuities. Therefore, a perforate bridge is made between the extended inlet and outlet. It is shown that the TL curve of a concentric tube resonator (CTR) can also be lifted in a similar way by proper tuning of the extended unperforated lengths. Differential lengths have to be used to correct the inlet/outlet lengths in order to account for the perforate inertance. The resonance peak frequencies calculated by means of the 1-D analysis are compared with those of the 3-D FEM, and appropriate differential lengths are calculated. It is shown how different geometric characteristics of the muffler and mean flow affect the differential lengths. A general correlation is obtained for the differential lengths by considering seven relevant geometric and environmental parameters in a comprehensive parametric study. The resulting expressions would help in design of extended-tube CTR for wide-band TL.
 
Article
In recent years, noise barriers are considered as one of the main ways to control traffic noise pollution. In this study, two different types of noise barriers: the parallel barrier model with primitive root diffuser (PP) and the parallel barrier model with quadratic residue diffuser (PQ) were designed, and the two-dimensional boundary element method was used to predict their efficacy in noise control. The results of this research compared the improvement in performance of parallel barriers covered with the primitive root diffuser (PRD) with parallel barriers covered with the quadratic residue diffuser (QRD) and the simple parallel barriers (P). Efficacy in different frequencies, using the diffuser with different depths based on the PP model was studied and was compared to the PQ barrier and a reference barrier (P) without any wells. The results showed that the efficacy of the parallel sound barrier arm covered by PRD in different intended receiver positions was higher than QRD and the simple parallel sound barrier arm.
 
Article
Improvements in methods of jet noise prediction and reduction are discussed. Measurements were performed on a cold-air jet exiting through convergent nozzles of varying diameters, and on superheated jet streams at an oil refinery. A critical value was determined for the stagnation pressure, beyond which the jet becomes choked and shock-cell noise occurs along with turbulent noise. The turbulent noise was isolated by making the lip of the jet irregular, then measuring the A-weighted sound level. Blow-down studies were also made of the shock-cell noise, which was found to happen above the frequency peak of the turbulent noise. The interaction of microjets grouped together is considered, noting that a form of the Coanda effect causes the formation of a single jet. A micropore muffler is presented, which consists of a pipe drilled with many holes, and a diffuse-muffler is mentioned, which contains randomly distributed holes internally to dampen the noise emission.
 
Article
According to some International and National Standards, frequency weighting is the preferred method for evaluation of human exposure to broadband vibration in buildings. The weighting functions are defined by prototype filters that may be implemented in analog or digital form. For this paper, frequency-weighting functions specified by the ISO 8041 and BS 6841 Standards were simulated in software as time-domain digital filters. Simulation procedures, software implementation, and verification of tolerance requirements are presented. The digital filters implemented here conformed closely to specifications in the Standards. A FORTRAN subroutine is provided for calculating the coefficients of the digital filters.
 
Article
A one-step gearbox was run under controlled laboratory conditions until failure occurred during continuous operation with an overload of about 50 percent. The gearbox ran for a total of 497 hours, until three teeth of the input gear broke at the base as a result of fatigue. Gearbox vibrations and temperatures were measured during operation. The following analyses of vibration signals were used for condition monitoring: spectrum analysis; cepstrum analysis; acoustic emission; and statistical analysis. The imminent failure could be predicted only marginally on the basis of spectrum analysis, but more accurately on the basis of cepstrum analysis. Under normal load conditions, a failure of this type might be predicted by cepstrum analysis several days in advance.
 
Article
In the study of product sound quality, most approaches used to analyze the result of the jury study assume that preference within the jury population is universal. This assumption holds true in some cases, but there are often scenarios where preference varies significantly with demographic. Identifying the presence of and partitioning the jury members with these varying criteria for preference is seldom a trivial exercise. For this reason, a method that can be used to evaluate a jury population and identify subgroups based on the juror voting patterns would be a useful tool. To address this problem, this paper presents an approach to infer the number of subgroups and to classify the jury members into the appropriate subgroups. The method is demonstrated in this paper using both a K-means clustering and a Ward’s clustering algorithm. In addition to providing a tool to improve the quality of the jury preference models, jury subgroups also have implications on brand imaging for target audiences. Identifying subgroups of consumers with differing preferences provides marketing groups with additional knowledge that can be used for targeted brand imaging.
 
Top-cited authors
Hugh W. Coleman
  • University of Alabama in Huntsville
Dietrich Schwela
  • Stockholm Environment Institute (SEI)
Nikolay D Gaubitch
Paolo Gardonio
  • University of Udine
Paul Schomer