Reverberation Room Method Research Articles

Reverberation room design optimisation for low-frequency diffuse sound absorption testing

The reproducibility of sound absorption testing with the reverberation room method is a long-standing concern. Absorptive samples induce directionality in the nearfield, while the farfield depends on the room geometry below the Schroeder frequency. Nevertheless, when properly accounting for nearfield effects, the theoretical diffuse absorption coefficient of a sample still represents its average performance across an ensemble of different rooms, even at very low frequencies. Recent research found that particular reverberation room designs allow for an accurate measurement of the diffuse sound absorption coefficient of highly absorptive samples at low frequencies. Pinpointing such designs hence opens up a possibility to sustainably improve the low-frequency reproducibility of sound absorption testing in reverberation rooms. The present paper introduces a numerical optimisation framework that serves this purpose. Specific room shapes are parametrised and the geometrical room parameters are optimised so as to minimise the difference between the measured and the diffuse absorption coefficient under appropriate constraints. The sound absorption testing of a sample in a particular reverberation room is numerically simulated using a method that is both accurate and computationally efficient at low frequencies. The diffuse absorption is computed with a hybrid deterministic-statistical energy analysis approach that accounts for the detailed absorber properties, geometry, and boundary conditions, as well as the nearfield effects. The methodology is applied to both cuboidal and hexahedral room shapes. Certain optimised designs are found not only to provide an excellent match for the absorber that was used during the optimisation, but they also maintain their performance across a range of absorptive samples. Additionally, potential geometrical deviations are found to be well tolerated by these reverberation room designs.

SOUND ABSORPTION COEFFICIENT MEASUREMENT METHODS IN REVERBERATION ROOM AND IMPEDANCE TUBE

Sound absorption materials are very often used for space design in order to reduce the noise level and adjust the acoustic characteristics of the space depending on its purpose. Therefore, knowledge of the acoustic quantities that characterize sound absorption materials is of crucial importance. The most commonly used acoustic quantity is the sound absorption coefficient, which can be determined using standardized methods and methods used only for research purposes. This paper will provide an overview of the most commonly used standardized methods for determining the sound absorption coefficient: the reverberation room method and the impedance tube method. Since the impedance tube method, which provides the normal incidence sound absorption coefficient, is more suitable when developing new absorption materials, and in practical applications, it is desirable to know the random incidence sound absorption coefficient, this paper discusses approaches for predicting the random incidence sound absorption coefficient based on the acoustic parameters of the sound absorption materials measured in an impedance tube.

Which absorption coefficients are correct?

Each year absorptive material samples are sent to various accredited acoustic laboratories around the country by manufacturers who want to verify the performance of their products. Results are published as indisputable validation of the product absorption performance by the manufacturer. As no recent interlaboratory study results for ASTM C423—Standard Test Method for Sound Absorption and Sound Absorption Coefficients by the Reverberation Room Method are available, the Owens Corning Acoustic Research Center initiated a private interlaboratory study. A small set of accredited acoustical laboratories tested a set of the original National Voluntary Laboratory Accreditation Program reference panels. The resulting data and differences in the measured absorption coefficients will be reviewed.

AN INVESTIGATION OF THE NOISE ABSORPTION POTENTIAL OF A SIX SELECTED GRASS SPECIES

Grass has a high leaf density and requires minimum space to grow. This experiment was designed to determine the sound absorption behaviour of six grass species (Zoysia matrella (L) Merr., Stenotaphrum dimidiatum(L.) Brongn, Panicum repens (L.), Eleusine indica (L.) Gaertn., Axonopus compressus (Sw) P. Beauv, and Ischaemum sp.) for their possible use as noise screens. The sound absorption of each morphological leaf structure was studied. For Sound Absorption Coefficients (SAC) (α) studies, the reverberation room method under ISO 345:2003 standards was followed. A B&K dodecahedron Omni-directional speaker, power amplifier, and 2250L handheld analyser were used for reverberation time and RT60 measurements. Microscopic images of grass leaves were analysed using ImageJ software. This study revealed that grasses with the highest and lowest SAC for higher noise frequencies (> 1500 Hz) are S. dimidiatum Brongn and A. compressus, respectively. The SAC of S. dimidiatum Brongn positively correlated with noise frequency. In general, the correlation of SAC (α) with noise frequency (f) is in the form of log10α = a1log10f + b1 where a1 and b1 are grass type-dependent constants. The morphological parameters like total leaf area, total sample area, plant height, and sample dry weight strongly correlated with the SAC. But leaf thickness, length, width, surface area, and the weight of the sample poorly correlated with SAC in the frequency range.

Tuning of Estimated Sound Absorption Coefficient of Materials of Reverberation Room Method

Acoustical materials are usually used for architectural design, ambient noise, and traffic noise to absorb sound. The sound absorption coefficient (SAC) is often used as the performance index of acoustical materials. It is measured using the reverberation room method (RRM) and the impedance tube method (ITM). The sound source of the RRM is a random incidence in various directions, whereas that of the ITM is a normal incident. The SAC measured by the RRM is more extensive, practical, and costly than ITM. Usually, the SAC measured by RRM is used as the commercially available acoustical materials index. However, RRM requires large-area test samples for measurement and a special sound field of reverberation room for test experiments. Therefore, this study aims to use the ITM to measure SAC. The measured SAC, frequency, thickness, and material density were employed as independent variables and substituted in the obtained multiple regression model to predict SAC obtained by RRM. In addition, the measurement characteristics of impedance tubes provide varisized calibers for different frequency ranges to measure a relatively accurate SAC. Based on the abovementioned characteristics, this study proposed a dual-model for estimating the SAC of the reverberation room method to achieve the best-estimated result. After experimental validation, the dual-model estimated result was compared with SAC from the reverberation room method. It was observed that the maximum absolute value of all errors did not exceed 0.16. Most of the absolute errors were below 0.1, proving the accuracy of this method. Therefore, the method proposed in this study can shorten the acoustical material development schedule for manufacturers and save the cost of the development process.

Calculation of statistical absorption coefficient using ensemble averaged surface normal impedance of material

To predict and control the indoor sound field, it is important to comprehend sound absorption characteristics of building materials. In the previous studies, the authors have proposed an in-situ sound absorption measurement method of materials using ensemble averaging technique, namely EA method. The method yields a simple and efficient in-situ measurement of surface normal impedance of materials at random-incidence. In this paper, the authors calculate the statistical absorption coefficient using the surface normal impedance of material by the EA method to obtain random incidence absorption coefficient. At first, the procedure of calculating the statistical absorption coefficient from the normal impedance by EA method is described. Next, the sound absorption characteristics for five kinds of materials are measured by the EA method and the reverberation room method. Finally, the statistical absorption coefficients are calculated from results obtained by the EA method and are compared with absorption coefficients by the reverberation room method.

Optimal Design and Test Study on Sound Insulation Property of Exterior Wall Enclosure of Prefabricated Substation

In this paper, based on the fabricated wall panel used in original modular indoor substation, the optimal design of sound insulation property is conducted according to the relevant theoretical basis of sound insulation properties of wall panels, and the reverberation room method is used to test and study original and new wall panels. The study shows that the sound insulation of the sandwich panel after optimization has been greatly improved in both medium, low and high frequency areas, with obvious optimal design effect, and also there are significant engineering application value and social benefits in the noise control of substation.

Research on the Effect of Imprecise Size Foam Samples on Acoustic Measurements in Impedance Tube

Polymer foam materials are very commonly used acoustic materials for absorbing sound energy in noise control engineering, because of their higher porosity and light in weight. The acoustic characterization of these materials namely; absorption coefficient and transmission loss are measured using impedance tube, reverberation room method and intensity methods. Among these three impedance tube is the most popular method in laboratory condition. Here the sample size plays a main role in the accurate characterization acoustic materials by using the impedance tube. The present paper discusses effect of variation in the sample size on their acoustic measurements. The foam sample are prepared using Resistance wire foam cutter(RWFC). Their absorption coefficient and transmission loss for large and small samples with variations in sample dimensions are measured and compare with ideal dimension results. A detailed measured data and analysis are done to understand the effect of undersized and oversized foam samples on absorption coefficient and transmission loss results as a function of frequency. The results show that the variations in sample size has immense effect of measured results. The results presented in this study becomes a guideline for characterizing acoustic materials using impedance tube.

Laboratory and in situ sound absorption measurement under a synthetized diffuse acoustic field

This article reports numerical and experimental results concerning the estimation of the diffuse field sound absorption coefficient of several different materials under a synthetized diffuse acoustic field excitation in laboratory and in situ conditions. The proposed measurement method is based on a sound field reproduction approach and a synthetic array of acoustic monopoles facing the material to be tested. Numerical simulations are first conducted to optimize the geometrical parameters of the method and to compute theoretical sound absorption coefficients of the considered materials. Measurements on a set of six typical acoustic materials are then conducted following the standardized reverberant room method as well as the proposed approach in a hemi-anechoic room and in two realistic rooms. Albeit showing limitations in the low-frequency domain, the proposed method enables a significant reduction of the tested specimen dimensions compared with the reverberant room method and allows performing tests in non-ideal acoustic environments.

A Basic Study on a Rectangular Plane Space Sound Absorber Using Permeable Membranes

In this communication, the sound absorption characteristics of rectangular-shaped plane space sound absorbers without any backing structure using permeable membranes (PMs) are measured by reverberation room method. First, three types of PMs, in this study woven fabrics, are selected with different flow resistances and surface densities. They are prepared in the plane rectangular-shaped space absorbers of two different sizes. The measured results are discussed through comparison with the existing theoretical and measured results for absorbers of the other shapes or configurations. The present results and discussion demonstrate that the reverberation absorption coefficients of the proposed absorbers are low at low frequencies and converge to a moderately high value at high frequencies. Especially, ones with higher flow resistance than the air impedance converge to a value greater than 0.5, which is a theoretically estimated maximum absorption coefficient of infinite single-leaf PM. This is inferred to be attributed mainly to area effect. From these results the proposed absorbers can be used effectively despite of their very simple structure. Also it is found that the proposed absorber can offer higher sound absorption than permeable membrane absorbers of other shapes or configuration. Regarding the effect of the size, the absorbers of smaller size offer higher absorption coefficients regardless of material properties of the PMs used in the experiments.

ESTIMATING SOUND ABSORPTION IN COEFFICIENT OF PRAYERS IN MOSCQUES

The article deals with analysing sound absorption of prayers in big congregational mosques. Peculiarity of sound absorption measurement in prayer halls is linked with the capability of mosque worshipers to pray in different poses with diverse arrangements as opposed to the traditional halls where people usually sit in chairs. This factor has a strong influence over the sound absorption coefficient which should be taken into account in acoustical design of such buildings. Within the framework of this study sound absorption coefficients (SAC) were measured using reverberation room method for people praying on carpets in different poses. Calculated data have been

Comparison the Sound Absorption Rate of Mandarin Peel – Sawdust Composite Boards Measured by Transfer Function Method with by Reverberation Room Method

Comparison the Sound Absorption Rate of Mandarin Peel – Sawdust Composite Boards Measured by Transfer Function Method with by Reverberation Room Method

A general description of measurement method for sound absorption and surface impedance of materials using ensemble averaging

Sound absorption characteristics of materials are important for acoustical researches as well as for practical designs. Various measurement methods have been proposed in this research work for sound absorption characteristics. Some of them are standardized in the International Organization for Standardization(ISO) and in the Japanese Industrial Standards (JIS). There are two well-known methods of laboratory measurement of absorption, one is the “reverberation room method” and the other is the “tube method”. However, these two methods have several drawbacks and disadvantages respectively, as they are not suitable for “in-situ” measurement. Furthermore, sufficient amount of impedance database has not yet been accumulated to meet the necessity for constructing boundary conditions of wave-based room acoustical simulations. To overcome the situation, the authors have presented a method utilizing “ensemble averaging”, namely the “EA method”. There are two types of EA methods. One uses two-microphones and the other uses a pressure-velocity probe (Microflown PU-sensor). Herein, an overview of the EA method is described.

Comparison of Transfer Function Method and Reverberation Room Method in Measuring the Sound Absorption Coefficient of Rice Straw Particle Mat

This study estimates the sound absorption capability of rice straw particle mats by two methods: the transfer function method and the reverberation room method. In the reverberation method, the central frequency was set to the one-third octave band in the 100-5000 Hz range; in the transfer function method, the frequency range was 500-6400 Hz. Both methods yielded similar noise reduction coefficients of the rice straw mats. The noise reduction coefficient was approximately 0.8, indicating a high sound-absorption property of the mats. Therefore, rice straw matting is a suitable candidate material for sound-barrier walls against highway noise.

DESAIN OF ACOUSTIC SANDWICH COMPOSITE FROM FOAM CONCRETE AND PALM FIBER

The purpose of this research were to investigate the influence of palm fiber composition and the number of hole cavities was varied with design Acoustic Sandwich Composite on sound absorption of acoustic panel. Acoustic panel were made of palm fiber as core and foam concreate as skin upper layer and bottom layer where there is a hole cavity on skin upper layer. Composition and the number of hole cavities was varied from 1 to 3 kg and the number of hole cavities from 25 to 81 hole. Sound absorption measurement was conducted by using reverberation room method based on ISO-354 (2003) with pink and white noise as sound source.The results showed that sample thickness affect acoustic absorption value effectively shifted at low frequencies, as in this research the best sound absorption occurs in panel with composition palm fiber most widely 3 kg, but coeffesient absorpsi value effective is in the range 250 Hz and use the number of hole cavities at skin upper layer of each sample effectively cause sound absorption coeffisient value increased, for both pink and white noise from sound source.Keyword : acoustic panel, foam concrete, palm fiber, sandwich composite, and absorption coefficient

In situ impulse response method of oblique incidence sound absorption coefficient with microphone array

The paper reports the development of the methods for measuring oblique incidence sound absorption coefficient in situ. The standing wave tube method and reverberation room method are not suitable for the test in situ. And the transfer function method presented by Allard fails to consider scattering effect of unsmooth surfaces. Based on the use of microphone array, the paper presents an impulse response method to study oblique incidence sound absorption coefficient. The impulse signal generator is designed to simulate pseudo-random noise including many different frequencies. The microphone array can eliminate the effect of reflection and scattering. The paper compares the impulse response methods of in situ measurement in reverberation room and free field, with another method which used in standing wave tube. The specimen is made of polyurethane foam. The results show that the absorption coefficient obtained by the standing wave tube is similar to reverberation room, but smaller than free field. Although the results obtained by three methods are not the same, the three curves reflect the same varying tendency. This in situ method can satisfy the technical requirement of measuring.

Signal processing method for compensating air absorption and phase change in room impulse response

It is often required to remove an influence of air absorption effect from a measured impulse response (e.g., absorption coefficient measurements by a reverberation room method). In particular, a room acoustic test in a scale model is greatly influenced by the air absorption and most of the attenuation of the observed impulse response is caused by air absorption, because the test measurement must be carried out in a high-frequency region including ultrasonic band in order to cover a wide frequency audible range in the real scale. Therefore, a high-accuracy compensation technique is needed so as to correctly evaluate the scale model responses. So far, two methods have been proposed, which utilize a subband division or a time-varying filter. In this study, we propose a new signal processing technique to remove the air-absorption effect by using a propagation distance distribution function obtained from the impulse response and a propagation constant. Some practical application results are presented and show that our proposed method can be performed with high accuracy and overcome some disadvantages of the existing methods.

An experimental study on the absorption characteristics of a three-dimensional permeable membrane space sound absorber

In this study, we propose a rectangular and cylindrical three-dimensional space sound absorber using a permeable membrane and the absorption characteristics which are examined experimentally by reverberation room method. As a pilot study, a two-dimensional boundary element (2-D BEM) analysis is also conducted to predict the absorption characteristics of the absorbers. The experimental study revealed that the absorption coefficient is low at low frequencies and gradually increases with frequency. The absorption coefficient converges to 0.5 at the maximum which is similar to a single-leaf permeable membrane. The flow resistance and the surface density of the permeable membrane mainly affect the absorption characteristics at middle to high frequencies. At low frequencies, the heavy membrane contributes to the higher absorption performance. In the experiment specimens with high flow resistance produce higher absorptivity. Also, the cylindrical absorber shows slightly higher absorption coefficient than the rectangular absorber mainly at low frequencies. The 2-D BEM results show similar frequency characteristics as the measured values when the membrane's flow resistance and surface density are low, but the numerical values overestimate overall the absorptivity of the absorbers.

Procedure for determination of sound power levels of direct-radiator loudspeakers in the low-frequency range using the sound pressure within the system enclosure

In this paper, a method of sound power level determination in the low-frequency range applied to direct-radiator loudspeakers and using the sound pressure within the system enclosure is proposed and discussed. The procedure is based on the Thiele-Small model of the loudspeaker, whereby the emitted sound power over a 2π sr solid angle is related to the total acoustic volume velocity radiated by the system, which simultaneously can be linked to the internal sound pressure in the box. Under normal conditions, this sound pressure is independent of the acoustic environment present outside the loudspeaker. To determine the internal sound pressure, the microphone could be easily inserted into the enclosure, usually through the bass-reflex opening in vented-box systems or via a small orifice in closed-box loudspeakers. The proposed method has been tested and compared with the standard methods used commonly nowadays (the reverberation room method and the anechoic room technique) on a bass-reflex loudspeaker system whose electrical analogous circuit model and sound power response are deeply known. To assess the results, the theoretical sound power response based on the electrical analogous circuit of the loudspeaker (Thiele-Small model) has been compared with all the former measurements, obtaining high agreement in the low-frequency range when the proposed method is compared with the sound power response of the theoretical model, thus validating the procedure. Even more, the measurements on the loudspeaker using the method here proposed seem to be better in the low-frequency range than that obtained by the standard methods, that is, the reverberation and the anechoic room techniques.

M113 車室空間における中周波数領域の新たな音響解析手法の研究(GS10 機械力学・計測制御(3),修士研究発表セッション)

M113 車室空間における中周波数領域の新たな音響解析手法の研究(GS10 機械力学・計測制御(3),修士研究発表セッション)