Scale Reverberation Chamber Research Articles

The effect of changes in atmospheric conditions on the measured sound absorption coefficients of materials for scale model tests

The absorption of scale model materials is generally measured in a scaled reverberation chamber, with frequency scaled according to the scale factor. Moreover, the acoustic absorption of the chamber needs to be reduced, especially the absorption of air. It depends not only on the volume of the chamber, but also on the air parameters: pressure, temperature and relative humidity. Most frequently, in order to reduce the air absorption, the air inside the model is dried or replaced with dry nitrogen, or a digital air absorption compensation is used. Air-drying methods are time consuming and require specialised equipment; digital compensation may in turn result in errors, hence the question arises whether scaling the air absorption is really necessary to achieve good accuracy of the sound absorption coefficient measurements. In order to answer this question, the authors firstly ran a theoretical analysis of the issue based on ISO 9613-1 formulas describing air acoustic absorption. Based on this analysis, the influence of particular air parameters on the total air absorption in the model reverberation chamber, Aair (m2), was examined, together with the possibilities of scaling down the Aair values. This indicated, that the Aair may be scaled by manipulating temperature or relative air humidity. For the needs of the study, the relative air humidity was chosen, as it was used by the predecessors and may be easily manipulated. In the next step, the experimental studies of sound absorption coefficient measurements were conducted in a 1:8 scale reverberation chamber for representative specimens at different relative air humidity values (3–45%). The gathered data was then statistically analysed. The results showed that the influence of relative air humidity on the accuracy of sound absorption coefficient measurements in model tests is negligible and therefore such measurements can be performed in the ambient air humidity of the test room. As a consequence, a conclusion was drawn that the Aair value does not need to be scaled. Moreover, an attempt was made to identify the cause of the inaccuracy of the measurements – the analysis indicated that the values of the intensity attenuation coefficient m calculated according to ISO 9613-1, at model frequencies are not sufficiently accurate.

Kurtosis of room impulse responses as a diffuseness measure for reverberation chambers.

This study presents a kurtosis analysis of room impulse responses as a potential room diffuseness measure. The early part of an impulse response contains a direct sound and strong reflections. As these reflections are sparse and strong, the sound field is unlikely to be diffuse. Such deterministic reflections are extreme events, which prevent the pressure samples from being distributed Gaussianly, leading to a high kurtosis. This indicates that the kurtosis can be used as a diffuseness measure. Two rooms are analyzed. A non-uniform surface absorption distribution tends to increase the kurtosis significantly in a small room. A full scale reverberation chamber is tested with different diffuser settings, which shows that the kurtosis calculated from broadband impulse responses from 125 Hz to 4 kHz has a good correlation with the Sabine absorption coefficient according to ISO 354 (International Organization for Standardization, Geneva, Switzerland, 2003).

Comparison of Hanging Panels and Boundary Diffusers in a Reverberation Chamber

Achieving a diffuse sound field in a reverberation chamber is crucial for measurements of acoustic quantities such as sound absorption coefficient, scattering coefficient, and sound power level. Toward this aim, diffusing elements such as hanging panels or rotating diffusers are typically installed in the chamber. However, previous research has suggested that hanging panels violate certain theoretical assumptions regarding diffusivity. Also, rotating diffusers cause the chamber to be a time-variant system, precluding the use of some measurement approaches such as sine-sweep integrated impulse response techniques. Boundary diffusers are offered as an alternative in the current study. The effects of both hanging panels and boundary diffusers on sound field diffusivity in a scale reverberation chamber are systematically and comparatively analyzed. The field diffusivity is characterized based on the guidelines set forth in American and international standards, including ISO 354, ASTM C423, and ASTM E90. Resulting data suggests that these standardized methods do not adequately or rigorously quantify diffusivity. The relative effectiveness of hanging diffusers vs. boundary diffusers will be discussed.

Effect of boundary diffusers in a reverberation chamber: standardized diffuse field quantifiers.

The sound field inside a reverberation chamber must have a high degree of diffusivity to allow for the accurate measurement of various acoustic quantities. Typically, hanging or rotating diffuser panels are installed in the chamber in an effort to achieve this diffusivity. However, both of these diffuser types have certain limitations, and adequate sound field diffusivity is often difficult to realize. A 1:5 scale reverberation chamber has been used to systematically analyze the relative effectiveness of hanging diffusers versus an alternative diffuser type referred to as a boundary diffuser. To characterize sound field diffusivity, three quantifiers from the ASTM E90, ASTM C423, and ISO 354 standards have been used: maximum absorption coefficient, standard deviation of decay rate, and total confidence interval. Analysis of the quantifier data reveals that boundary diffusers and hanging diffusers produce roughly equivalent diffusion in the sound field. The data also show that the standards have certain inconsistencies that can obfuscate the characterization of sound field diffusivity, which may explain reproducibility and repeatability issues previously documented in the literature.

Effect of boundary diffusers in a reverberation chamber: A preliminary investigation

In this project, the sound field behavior in a 1:5 scale reverberation chamber has been measured and analyzed. Both hanging diffusers and boundary diffusers have been applied in an effort to increase the chamber’s sound field diffusivity, which has been characterized based on the guidelines set forth in several American and international standards, including ASTM C423, ASTM E90, and ISO 354. Objective data from measured impulse responses for several configurations of the diffusers will be presented. These data will be compared to those from the empty chamber and to the criteria from the standards. The relative effectiveness of hanging diffusers vs. boundary diffusers will be discussed.

Measurements of the acoustical properties of iron slag panels as porous media using scale models

Iron slag, by-product of smelting iron ore, has been widely recycled to road pavement material and concrete for environmental-friendly effect. In the present study, the acoustical characteristics of iron slag panels (porous media) were investigated by using scale model approach. The scale models of the iron panels were constructed by considering different sizes of slags. The measurements of absorption and scattering/diffusion coefficients using scale models were conducted in a 1:10 scale reverberation chamber. Through the measurements, the effects of diameters and thicknesses of slags on acoustical properties of iron slag panels were investigated.

소형잔향실을 이용한 고속철도 차량 구조재의 투과손실 연구

The method to reduce interior noise of train was being studied. To improve transmission loss of train is one of the best way to reduce interior noise. But, the estimate to transmission loss requires lots of the commercial costs. In this study, the method to estimate transmission loss of high speed train is proposed using a scale reverberation chamber. The result shows that a transmission loss estimated using small scale reverberation is similar to that using huge reverberation chamber. The transmission loss estimated based on small scale reverberation chamber can be optimal with respect to the commercial coasts.

Experimental quantification of a novel reverberation chamber diffusing panel.

In this project, a 1:5 scale reverberation chamber was designed, built, and tested. The envelope geometry of the chamber was chosen to produce an isotropic sound field and uniform modal response. To further scatter the sound energy, several randomly placed reflective panels were installed in the room. Additionally, a novel diffuser panel with resonating properties was characterized and utilized in the chamber design. The properties of this diffuser/resonator will be discussed. Objective data from measured impulse responses for several configurations of the reflective panels and diffuser/resonators will be presented. These data will be compared with minimum reverberation time (RT) values specified in ISO 345. The level of diffusivity will be shown using the RT standard deviation between receivers as specified in ASTM C423. Finally, the modal response of the chamber will be discussed. These data were analyzed to determine the appropriate number of reflective panels and optimum panel configuration. An overview of this optimization process will be presented. A computer model of the chamber was also analyzed in ODEON; computational and measured results will be compared and contrasted. [Work partially funded by the ASA Diversity in Acoustics Committee and the National Society of Black/Hispanic Physicists.]

Design of the diffuser with hemispheres using 1:10 and real scale models

Sound diffusion by a wall structure is one of the main concerns with respect to the sound quality of concert halls. To quantify the sound diffusion of an actual hall from in situ measurements, the relationship between the scattering coefficient and the acoustical parameters measured in the laboratory should be investigated first. In this study, the scattering coefficient and acoustical parameters of the diffusers with hemisphere were investigated. The diameter, density, and the arrangement pattern of the hemisphere were investigated in terms of the scattering coefficient and the parameters obtained from the impulse response including temporal diffusion [H. Kuttruff, Room Acoustics (Elsevier Science, London, 1991)]. The results of measurements in 1:10 scale reverberation chamber with the same scale diffusers were verified by real scale results. Sound scattering index for reverberation chamber, which quantifies the scattered sound field, was proposed. The optimum condition of the hemisphere diffuser specification maximizing the effect of scattering on a sound field was also investigated.

Reduction of acoustic radiation by perforated board and honeycomb layer systems

A perforated system, proposed previously for reducing the radiated sound from a plate at arbitrary frequencies, is applied to three-dimensional problem. Plates are assumed to be supported in a duct of a finite cross-section and excited by a harmonic point force. The sound radiation is investigated from the viewpoint of acoustic power and it is discussed whether the attenuation effect shown previously in the one-dimensional system can be obtained with the three-dimensional system. The effect of support conditions on attenuation characteristics is discussed by using clamped and simply supported circular models. Allowing for the effect, a simply supported rectangular model is studied in detail and its problems are revealed. In order to overcome the problems, a new system including subdivided air cavities in the form of a honeycomb layer instead of a undivided backing cavity is proposed. Each of the honeycomb cells can create local one-dimensional sound fields. Calculated theoretical results are compared to data obtained in a 1/5th scale reverberation chamber. The results for the reduction effect, which are in good agreement, show that the honeycomb layer system can achieve the same reduction of the radiated sound power at arbitrary frequencies as the one-dimensional perforated system.

Sound absorption in a scale model reverberation chamber

Scale models of rooms are widely used to investigate architectural acoustics. However, when building a room model, the choice of materials to simulate the surfaces of the original room should follow carefully the similarity conditions. In practice, it is very difficult to model the acoustic parameters of the materials used in room scale models. This work consists in the construction and validation of a small-scale reverberation chamber, according to the international standards, developed in order to improve similarity conditions in scale modeling. Such a chamber is relatively low cost and its configuration can be easily modified. The volume of the model reverberation chamber is 0.389 cubic meters, equivalent at 1:8 scale to a real volume of 200 cubic meters, a standard size for reverberation chambers. Made in wood with one transparent acrylic side, the one-eighth scale reverberation chamber can be used to test theories about room acoustics as well as to test absorbing materials. Experimental results for the chamber qualification are presented and the search for octave band similar materials is reported.

Precision Measurement of Acoustic Impedance of Sound Absorbing Materials

A brief survey is made of the limitations of existing methods for the measurement of the normal acoustic impedance of sound absorbing materials. A more exact method has been developed which utilizes a cylindrical tube terminated at one end by a rigid wall at whose center the sound is admitted and at the other end by a movable piston on which the absorbing sample may be mounted. The acoustic impedance is computed from the widths and positions of pressure-length resonance curves, the pressure being measured at the tube wall at the source end. An exact mathematical analysis is provided which includes correction for the effects of dissipation in the tube and distortion of the sound field in the vicinity of the source. In the frequency range of 100 to 8000 cycles, using two tubes of different diameters, measurements are made to show the various types of impedance curves available for practical applications, and to show variations between different samples of the same material. The effect of sample mounting conditions is discussed in connection with the correlation of small tube and full scale reverberation chamber measurements.