Reverberation Chamber Research Articles

Sound Power Measurement of Tyre/Road Noise Using the Close-Proximity (CPX) Trailer Enclosure

The present paper reports a new methodology for measuring the sound power level (SWL) of tyre/road noise, using a modified Close-Proximity (CPX) trailer enclosure. Specially designed diffuser panels are implemented to cover the absorption materials on the interior walls and ceiling of the CPX enclosure to create a reverberant chamber. Through numerical simulations, the optimal reverberant chamber design, which holds the least acoustic resonance and most uniform acoustic energy distribution, is determined to guide fabrication. The SWL of a reference sound source (RSS) is determined first. The sound pressure level (SPL) distributions inside the reverberant chamber with the RSS switched on are then obtained under static conditions and road tests. The relationships between the SWL of the RSS and the SPL it produces inside reverberant chamber with various RSS inputs signals show great consistency with theory. Using the linear regression of SWL with respect to spatially averaged SPL reverberant chamber obtained in static condition, the total SWL and its spectrum at a specific vehicle speed can be deduced from the captured SPL information inside the chamber during road tests with the RSS switched off.

Diffusion equation-based modelling of reverberation chambers for sound absorption measurements

Recent research in reverberation chambers has revealed unacceptably large variations resulting from round-robin tests of the standardized measurements of random-incidence absorption coefficients. These variations have prompted active research activities in architectural acoustics. This study applies a diffusion equation model (DEM) in acoustics of reverberation chambers for random incidence absorption measurements by predicting both reverberant sound energies and energy flows. The DEM is of high computational efficiency when compared with other existing geometrical acoustics and wave-based methods. The DEM modeling of the sound energy flows in a reverberation room further reveals violations of the diffuse field assumption near the boundary area of absorbing sample under test. Due to its computational efficiency and higher order of the reverberant energy prediction of the statistical room-acoustic modeling, this work investigates an estimation approach to the absorption coefficients of the sample under test. The proposed approach estimates the absorption coefficients by pursuing both the DEM simulations and the chamber-based measurements. This paper investigates sound energy flux over varied absorbing degrees of absorber samples in standardized sizes, computational efficiency of the DEM for reverberant energy decays in reverberation chambers, and potential significance applied to random-incident absorption coefficient measurements.

Hand-made sound absorber using familiar materials for nursery facilities

We have been proposing a hand-made sound absorber using familiar materials to develop a comfortable sound environment for children. By installing the hand-made absorber, anyone can easily add sound absorption to nursery rooms. In this study, as one of the familiar materials, we focused on bubble wrap, which is used as a packaging material. We measured the normal incidence absorption coefficient of bubble wrap arranged in layers and found that bubble wrap had reasonable sound absorption. As an example of a hand-made absorber, we created round-shaped fabric balls filling bubble wrap and measured the sound absorption performance in a reverberation room. As a result, the fabric ball absorber had sufficiently high sound absorption performance. Additionally, we clarified the sound absorption mechanism of bubble wrap by predicting based on simple assumptions. Therefore, it could be possible to design sound absorbers with higher sound absorption performance at low frequencies by stacking bubble wrap horizontally or vertically.

The effect of furniture on room acoustic parameters and its dependence on different suspended ceilings' sound absorptive properties

In ordinary public rooms such as classrooms and offices, the base line of acoustical treatment is an absorbent suspended ceiling. Due to the non-uniform distribution of the absorptive material, the scattering as well as the absorption properties of furniture will have a significant influence on room acoustic parameters such as reverberation time, speech clarity and sound strength. In particular, the absorption of the sound scattered energy will depend on the absorbing efficiency of the suspended ceiling. This effect is not accounted for in classical diffuse field models such as Sabine's formula. Measurements were conducted in the reverberant room with varying degrees of furnishing for several ceiling treatments ranging from highly reflective to highly absorptive. Using Sabine's formula and the Statistical Energy Analysis model, this paper discusses the scattering effect of furniture on room acoustic parameters such as reverberation time, speech clarity and sound strength as well as how the equivalent scattering absorption area is related to the absorbing efficiency of the suspended ceiling. It is concluded that a novel method for the quantification of furniture is needed to include both absorption and scattering effects of furniture on reverberation time and its dependence on the suspended ceilings' absorptive properties.

An adaptive multichannel speech dereverberation approach based on robust estimation

Room reverberation usually produces adverse influence on the quality of speech in human-machine interaction scenarios. To improve the clarity and intelligibility of reverberant speech, an adaptive multichannel speech dereverberation approach is proposed in this work. Unlike the traditional least-squares cost function for speech dereverberation, a robust estimator with an adaptive parameter is adopted to define a flexible cost function of the adaptive speech dereverberation algorithm. As a consequence, the developed adaptive filtering algorithm can dynamically track the non-stationarity and non-Gaussian properties of speech so that the reverberation component of speech is effectively suppressed. Numerical simulation experiments validate the dereverberation performance of the proposed approach.

A finite element study of absorption coefficient measurement at low frequencies

The frequency-dependent absorption coefficients of an acoustically absorbent material sample can be measured using the reverberation chamber method, defined by the ISO standard 354:2003. The lower frequency bound for the measurement defined in the standard is 100 Hz, since measurement data below this frequency may be unreliable. However, the ability to measure absorption coefficients at low frequencies is becoming increasingly more important, to facilitate the enhanced control of low frequency sound fields. This paper presents a finite element method study of the reverberation chamber measurement method at frequencies below 100 Hz, and the predicted performance of the ISO standard method is reported. Additionally, a recently proposed method that makes use of finite element-based eigenvalue analyses to estimate the locally reacting complex-valued impedance of a sample at modal frequencies is applied to the problem. It is shown that the proposed method can be used to estimate frequency-dependent absorption coefficients at modal frequencies below 100 Hz.

A basic study on laboratory measurement of oblique-incidence sound transmission loss of building elements

Understanding the sound insulation performance of building elements under oblique incidence conditions is occasionally required for assessing the indoor sound environment of high-rise buildings that face roads or railways. In order to gain knowledge toward establishing a laboratory measurement method for oblique incident sound transmission loss, a measurement system was set up in a coupled reverberation room and anechoic chamber with a movable sound source. For verification of the system, a pilot measurement was carried out with a large glass pane, while the experimental set-up was also numerically simulated by FEM. In the experimental results, transmission loss was observed to depend on incidence angle in correlation with the coincidence effect, roughly obeying theory. However, it was found that the incident power estimated from the surface sound pressure on the specimen differs somewhat from that estimated using loudspeaker directivity in a free field. This discrepancy is thought due to edge diffraction from the beading around the glass pane, and this was confirmed by the numerical results. Further, this edge diffraction may also have caused the observed coincidence effect under normal incidence.

Estimation of the sound power of multiple sources using SPL measurements and room acoustic simulations

The sound power of sources is a key input to room acoustic simulations. However, existing measurement methods are generally unpractical as they require specific equipment, for instance a reverberation room in ISO 3743. A method is proposed to find the sound power spectrum of sources for room acoustic simulations, based on in situ sound pressure level (SPL) measurements at known locations. The measured data is fitted to an ODEON simulation of the measurement space via an optimization algorithm. The method requires an accurate numerical model of the environment. A notable advantage is the possibility of estimating multiple sources at once with the same set of SPL measurements. The proposed method is investigated in a dry room in which up to three sources are active simultaneously. It is found that at least three receivers per source are required to obtain robust estimations, and it is beneficial to place them in the vicinity of the sources. The accuracy of the estimation also depends on the relative level between the sources, as a too strong source will mask a weaker one. Finally, dry environments are more suited to measuring multiple sources, due to larger spatial variations in SPL.

Experimental study on the sound of water flowing through a storm drain

In buildings where storm drains are installed indoors, the sound of running water radiated from the surface of storm drains during rainfall may raise the noise level in rooms near the drain. To take appropriate measures against such sound according to the required quietness for the room, quantitative data on the sound of running water from storm drains is necessary, but few studies have dealt with this kind of sound. Therefore, the authors conducted experiments in an existing building, a drainage test tower, and a reverberation room to understand the characteristics of the sound produced by storm drains. Based on the results obtained through these experiments, this paper presents the effects of flow rate, pipe diameter, pipe material, and horizontal piping on the sound of running water from storm drains. The paper also discusses the differences between the sound generated by storm drains and the sound generated by sanitary sewers.

Assessment of acoustical materials sound absorption coefficient under oblique incidence plane wave and diffuse field using a virtual source antenna

The measurement of the absorption coefficient of acoustic materials is usually performed in an impedance tube under normal incidence on a small sample, or in a reverberant room under diffuse field incidence on a large sample of material. Both methods are prone to well-documented experimental limitations and errors. The approach proposed here uses a virtual source antenna (a point source moved at successive positions) over a material sample typically 1 to 4 m2 in size, and a fixed microphone pair directly above the material surface. Several methods have been tested to reconstruct the absorption coefficient under oblique plane wave or diffuse field incidence. Among these, the inversion of the Allard propagation model over an absorbing plane, in order to extract the material properties (complex wavenumber and density) from measured microphone transfer functions, is presented here. Absorption coefficients are shown for various acoustic materials.

Estimation of the random incidence sound absorption coefficients of different size rectangular samples.

The sound absorption coefficient (SAC) of materials measured in a reverberation room is affected by both the intrinsic properties of the material and geometrical dimensions of the sample. A different size of the same material may produce a different SAC primarily due to the edge effect phenomenon. In this research, the experimental data from multiple laboratories was analyzed to evaluate the influence of the edge effect. An empirical function was established based on these measurement data and the linear relationship between the SAC and the relative edge length. Thomasson's method, the two geometric methods, and the analytical method were used to estimate the SAC of an absorber from measurements on a different size sample and compared with results obtained using the empirical function. The results show that the proposed empirical method is a reliable way to predict the SAC of a sample from measurements on a different size sample of the same material, which only requires the thickness, density, and size of the material.

Numerical Derivation of Design Guidelines for Tightness and Shaking Amplitude of Vibrating Intrinsic Reverberation Chamber by Method of Moment

Vibrating intrinsic reverberation chamber is being used as an in-situ EMC test equipment for large and complex systems such as automobiles and aircrafts. In this paper, the stirring conditions, such as tightness and shaking amplitude of the walls, of a vibrating intrinsic reverberation chamber have been analyzed using the method of moments. From the viewpoint of quantitative evaluation of the flexible moving walls configuration, it was found that the random electromagnetic environment can be generated under the stirring conditions of loose configuration and a shaking amplitude more than one eighth of the wavelength at the test frequency above the lowest usable frequency.

On the Cancellation of Unstirred Paths in a Fully Stirred VIRC Working as a Perfect Chaotic Reverberation Chamber

On the Cancellation of Unstirred Paths in a Fully Stirred VIRC Working as a Perfect Chaotic Reverberation Chamber

Accounting for Vibration Loads in the Design of Acoustic Reverberation Chamber Structures in the Midas FX Software Package

This article outlines the main provisions in the design of an acoustic reverberation chamber, methods for modeling such objects in engineering and construction practice. A finite element model of an object is presented in the Midas FX software package. As a result of the calculation, it is possible to evaluate the components of the stress-strain state in concrete and in structural elements or on the contact surface, the effect of vibration loads on RAC structures, their interaction with the foundation and soil base, as well as analysis of the use of vibration isolators in the RAC foundation Based on the results of the analysis, recommendations are given for perform calculations of system elements, select design solutions.

Deep encoder/decoder dual-path neural network for speech separation in noisy reverberation environments

In recent years, the speaker-independent, single-channel speech separation problem has made significant progress with the development of deep neural networks (DNNs). However, separating the speech of each interested speaker from an environment that includes the speech of other speakers, background noise, and room reverberation remains challenging. In order to solve this problem, a speech separation method for a noisy reverberation environment is proposed. Firstly, the time-domain end-to-end network structure of a deep encoder/decoder dual-path neural network is introduced in this paper for speech separation. Secondly, to make the model not fall into local optimum during training, a loss function stretched optimal scale-invariant signal-to-noise ratio (SOSISNR) was proposed, inspired by the scale-invariant signal-to-noise ratio (SISNR). At the same time, in order to make the training more appropriate to the human auditory system, the joint loss function is extended based on short-time objective intelligibility (STOI). Thirdly, an alignment operation is proposed to reduce the influence of time delay caused by reverberation on separation performance. Combining the above methods, the subjective and objective evaluation metrics show that this study has better separation performance in complex sound field environments compared to the baseline methods.

Experimental study on the sound transmission loss suite at the University of Technology Sydney

Sound transmission loss suites are essential testing facilities for measuring the sound insulation properties of building elements and assessing noise attenuation. However, inconsistencies in test results can arise due to variations in the size, shape, and construction of test rooms across different laboratories, with biases introduced by the room acoustics or acoustical environment of the facility itself. To evaluate the reliability of such testing, we conducted an experimental study on the sound transmission loss suite at the University of Technology Sydney. Our investigation focused on three key factors: estimating the maximum achievable sound reduction using a heavyweight wall installed at the test aperture on the source room side, testing the effectiveness of vibration isolation between reverberation rooms, and assessing the decoupling of the sound field within the reverberation rooms.

Time reversal acoustic focusing in a reverberant room: Elongating a focus, and the effect of head-scattering

Time reversal acoustics (TRA) has tremendous potential for focusing, or sound concentration, in reverberant architectural environments. While TRA provides an inherently simple means of achieving a high degree of spatiotemporal focusing, its appropriateness for listening applications in the built environment is challenged by the need to accommodate a person within a sufficiently large focal area. At its simplest, a reciprocal time reversal focusing system for audible range sound will produce a focus with a spatial spread corresponding to one-eighth of the wavelength, which is inconveniently small for speech frequencies. However, an elongated focus is introduced by deploying a directional impulse response, which can be created with a directional microphone, or synthesized by time-delay-mixing of multiple omnidirectional impulse responses. This paper reports on results of physical experiments conducted in a reverberant room, complemented by finite-different time-domain simulations in which the effect of introducing a head-sized scatterer was investigated.

Sound absorption coefficient measurement for various building elements

The current sound absorption coefficient testing measurement method, ISO 354, primarily focuses on materials used for surface coverings or equivalent sound absorption objects. However, there are other building elements such as lightweight partition walls and windows that do not have solid structures behind them. These elements may allow sound to transmit to adjacent interior spaces or escape to the outside. On the other hand, the transmitted sound is expected to be rebounded into the measurement room when the test material is placed on a hard and thick surface as described in ISO 354. The test was conducted in a reverberation chamber located at the center of three consecutive acoustic testing chambers: reverberation-reverberation-anechoic. The testing chambers are equipped with test openings between them and are fully isolated from one another. The purpose of the test was to confirm whether the current measurement method reflects the influence of the assumed rebounded sound. The sound absorption coefficients were measured with the specimen installed on the floor, representing the ISO 354 condition, and comparisons were made to the conditions where the specimen was installed in another reverberation chamber opening and in the anechoic chamber opening.

When less is more: Reducing environmental noise from open cycle gas turbines with acoustically transparent stacks

Sound radiation from large open cycle gas turbine power stations is strongly refracted by high temperature and velocity gradients in the near field of the exhaust stack outlet. Recent research by the authors has shown that in the presence of a cross-flow, the exhaust plume bends downwind and in doing so leads to significant increases in far-field sound pressure levels of up to 10dB compared to levels predicted from spherical spreading. This paper is the second in a series that explores a unique “acoustically transparent silencer,” which limits sound refraction consequently reducing downwind SPLs by separating the sound from the bulk exhaust flow. The efficacy of the approach is demonstrated two ways; initial characterization studies conducted in the reverberation chambers at the newly renovated Acoustic and Vibration Laboratory, University of Adelaide, and subsequent trials undertaken on a laboratory scale of 250 kW gas turbine in the field. Numerous designs are explored to quantify the effective insertion loss, where it will be shown as the transmission loss of the stack walls is reduced, the effective insertion loss increases, thus demonstrating that less is more. Reductions in ground-plane SPLs of up to 10 dB are achieved with less material and weight compared to traditional hard-walled stacks.

Listening to the room: Disrupting activity of dorsolateral prefrontal cortex impairs learning of room acoustics

Navigating complex sensory environments is critical to survival, and brain mechanisms have evolved to cope with the wide range of surroundings. In noisy spaces listeners place more emphasis on early-arriving sound energy, nevertheless, reverberant energy is highly informative about those spaces per se, and human listeners show improved speech understanding when re-encountering known reverberant environments. We assessed the ability of listeners to perceive speech (Coordinate Response Measure corpus) in noisy and reverberant rooms. We mimicked the acoustic characteristics of real rooms using loudspeakers positioned within an anechoic chamber. Listeners were also exposed to repetitive transcranial stimulation (rTMS) to disrupt the dorsolateral prefrontal cortex activity (DLFPC), a region believed to play a role in statistical learning. Our data suggest listeners rapidly adapt to statistical characteristics of an acoustic environment to improve speech understanding. This ability is impaired when rTMS is applied bilaterally to the DLPFC. The data demonstrate that speech understanding in noise is best when exposed to a room with reverberant characteristics common to human-built environments. Our findings provide evidence for a reverberation “sweet spot” and the presence of brain mechanisms that might have evolved to cope with the acoustic characteristics of listening environments encountered every day.