Reverberation Chamber Measurements Research Articles

Estimation of room absorption of acoustic cones based on impedance tube measurements

Acoustic cones are often the primary absorption treatment choice for constructing anechoic chambers. Reverberation chamber estimation of room absorption will most accurately represent the actual effect of three-dimensional treatments, such as cones. However, for low frequencies, large reverberation chambers are often required, which may not be available due to space and cost constraints. Impedance tubes measurements are more accurate than reverberation chamber measurements in terms of sound absorption coefficient. Conversely, due to the two-dimensional nature of impedance tubes, room absorption of three-dimensional objects is difficult to estimate. Absorption coefficient of the material is a function of not only surface area but also material thickness. This paper proposes a technique to estimate the room absorption of three-dimensional treatments based on measured absorption coefficient of different thickness treatment specimens by impedance tube. This extrapolation of impedance tube measured absorption coefficient to actual room absorption can be used to evaluate the effectiveness of absorptive acoustic room treatments, including effects of adhesives and air gaps, without the need of constructing the actual treatment prototypes. The validity of this impedance tube room absorption estimation was verified for a tetrahedral cone treatment using a large reverberation chamber room absorption measurement.

Radiation efficiency, correlation, diversity gain and capacity of a six-monopole antenna array for a MIMO system: theory, simulation and measurement in reverberation chamber

A six-monopole circular antenna array for use in a MIMO system is considered. The authors show how to calculate the embedded element patterns, both by classical analytical modeling and by the method of moments. Thereafter, these are used to calculate the radiation efficiency of each embedded element, correlation and diversity gain, as well as the maximum average capacity of the MIMO system when the array is located in a rich scattering environment. The theoretical value for the capacity is obtained by numerically distributing many plane wave sources statistically uniformly over 4π, letting them illuminate the calculated embedded element pattern and using Shannon's capacity formula on the received wave amplitudes. The calculated results are compared with measurement in a reverberation chamber, representing a similar scattering environment. The agreement is good.

Models for the number of independent samples in reverberation chamber measurements with mechanical, frequency, and combined stirring

The accuracy in reverberation chamber measurements relies on the chamber setup being able to generate a large number of independent measurement samples. Simple models for the number of independent samples with mechanical stirring and frequency stirring are presented, and also with several stirring techniques combined. The models are verified through measurements.

Reciprocity in reverberation chamber measurements

Even though reverberation chambers have been used primarily for radiated immunity testing, they are reciprocal devices that are equally applicable for radiated emissions testing. This short paper presents the theory for radiated emissions testing and uses electromagnetic reciprocity theory to demonstrate the link between radiated emissions and immunity testing.

A model for the number of independent samples in reverberation chambers

AbstractAntenna efficiency measurements in reverberation chambers are done by averaging a number of sampled power levels, measured while the mode stirrer rotates. The number of independent samples is the number of stirrer positions that give uncorrelated fields in the chamber, and thereby uncorrelated power levels. A larger number of samples gives a better accuracy of the measured value. This report presents a proposed theoretical model for the number of independent samples of a reverberation chamber, thereby giving a better understanding of how to optimize a chamber design. Measured data from chambers with three different types of stirrers are compared with the use of the model. © 2002 Wiley Periodicals, Inc. Microwave Opt Technol Lett 33: 25–28, 2002; DOI 10.1022/mop.10220

Microwave field-to-wire coupling measurements in anechoic and reverberation chambers

Knowledge of the differences in radiated susceptibility (RS) testing in anechoic (AC) and reverberation chambers (RC) is essential for analysis of the susceptibility of electronic systems to microwave radiation. We have studied microwave field-to-wire coupling for some basic wire geometries above a ground plane in ACs and RC. Results for receiving parameters such as the antenna receiving cross section /spl sigma//sub w/ and the effective antenna length of the wire, h/sub e/, are presented. The ratio between the maximum and average values of /spl sigma//sub w/ may exceed 15 dB in the AC, the average being equal to /spl sigma//sub w/ measured in the RC. Large variations in /spl sigma//sub w/ show that the outcome of an RS test in an AC may depend strongly on the direction and polarization of the incident field. For a realistic RS test in ACs only a few angles of incidence can typically be afforded, implying a substantial risk for undertesting. Furthermore, /spl sigma//sub w/ measured in the RC follows a /spl chi//sup 2/-distribution with two degrees of freedom. The measurements in the AC do not follow the same distribution.

Reflective antenna efficiency measurements in reverberation chambers

AbstractTraditional reverberation chamber antenna measurements consist of transmission measurements between a fixed antenna inside the chamber and the antenna under test. A novel method for measuring antenna efficiency is presented, in which only the reflection coefficient of the antenna under test is measured. Measurements indicate that the method is accurate. © 2001 John Wiley & Sons, Inc. Microwave Opt Technol Lett 30: 332–335, 2001.

MEASUREMENTS OF THE ABSORPTION BY AUDITORIUM SEATING—A MODEL STUDY

One of several problems with seat absorption is that only small numbers of seats can be tested in standard reverberation chambers. One method proposed for reverberation chamber measurements involves extrapolation when the absorption coefficient results are applied to actual auditoria. Model seat measurements in an effectively large model reverberation chamber have allowed the validity of this extrapolation to be checked. The alternative barrier method for reverberation chamber measurements was also tested and the two methods were compared. The effect on the absorption of row–row spacing as well as absorption by small numbers of seating rows was also investigated with model seats.

Definition and measurement of random-incidence scattering coefficients

This paper describes the definition and two methods for measuring scattering coefficients of rough surfaces. In both cases, impulse responses are determined for various orientations of a sample surface. After phase-locked superposition of a sufficient number of impulse responses, the coherent reflected sound energy is obtained. It is identical with the zero-order lobe of the reflection pattern — the specularly reflected part. Considering the totally reflected sound energy, the scattering coefficients can be calculated. The results of the reverberation chamber measurements are scattering coefficients for random sound incidence. Both methods are discussed and experimental results, obtained by scale model measurements, are shown.

Radiated emissions and immunity of microstrip transmission lines: theory and reverberation chamber measurements

The increasing complexity of electronic systems has introduced an increased potential for electromagnetic interference (EMI) between electronic systems. We analyze the radiation from a microstrip transmission line and calculate the total radiated power by numerical integration. Reverberation chamber methods for measuring radiated emissions and immunity are reviewed and applied to three microstrip configurations. Measurements from 200 to 2000 MHz are compared with theory, and excellent agreement is obtained for two configurations that minimize feed cable and finite ground plane effects. Emissions measurements are found to be more accurate than immunity measurements because the impedance mismatch of the receiving antenna cancels when the ratio of the microstrip and reference radiated power measurements is taken. The use of two different receiving antenna locations for emissions measurements illustrates good field uniformity within the chamber.

An overview of the acoustical effects of an audience.

This paper reviews the various effects of an audience on the acoustical properties of an auditorium. At the most basic level, the total sound absorption of both occupied and unoccupied chairs can be predicted in terms of the perimeter/area ratio of audience seating blocks. Using this technique, reverberation chamber measurements of occupied chairs can be used to estimate the reverberation times of fully occupied halls. This technique has been extended to provide estimates of typical sound‐absorbing properties of occupied chairs for all sizes of seating blocks. Partially occupied seating areas present further problems. Audience effects on early‐ and late‐arriving sound levels and early/late sound ratios are related to the geometry of the halls as well as to changes in the total sound absorption. Interaural cross correlations and lateral energy fractions are not greatly influenced by the presence of an audience. Changes to the more detailed spectra of the direct and early arriving sound show that the effect of the audience on the seat dip attenuation can vary with the type of seating as well as other properties of the hall.

Reply to: ‘‘Comments on ‘Predicting theatre chair absorption from reverberation chamber measurements’ ’’ [J. Acoust. Soc. Am. 93, 2238–2240 (1993)

Sound absorption coefficients of samples of theatre chairs are linearly related to the sample perimeter/area ratio due to both edge absorption and diffraction effects. The method of using screens to minimize the edge absorption of samples of theatre chairs may be practically useful, but is subject to experimental uncertainties. Predicting the effect of theatre chairs in an auditorium by extrapolating from the measurements of several different sized samples of chairs is still thought to be the most accurate approach.

Comment on ‘‘Predicting theater chair absorption from reverberation chamber measurements’’ [J. Acoust. Soc. Am. 91, 1514–1524 (1992)

The relationship between the measured acoustic absorption coefficient of an array of theater chairs and the ratio of the perimeter length to plan area of the array is considered. It is shown that the linear relationship measured by Bradley in a reverberation chamber and reported in ‘‘Predicting theater chair absorption from reverberation chamber measurements’’ [J. Acoust. Soc. Am. 91, 1514–1524 (1992)] is to be expected from simple theory. This means that any nonlinear influence of diffracted energy on the relationship is small. Bradley is also unduly harsh on the usefulness of a chair absorption measurement method involving screens. A sample result is given, showing that this method can predict in-situ theater chair absorption with reasonable accuracy.

Predicting the absorption of pew cushions

Previous research has shown that the sound absorption of arrays of theater chairs is related to the ratio of perimeter-to-area of a sample of chairs [J. Acoust. Soc. Am. 91, 1514–1524 (1992)]. Measurements of various sized samples in a reverberation chamber can thus be used to predict the expected absorption of a large array of chairs in an auditorium. This paper examines the related problem of predicting the absorptive effect of adding cushions to wooden church pews. The absorptive effect of the pew cushions was measured both in a reverberation chamber and in a church. Tests in the reverberation chamber included various sized samples of pews both with and without the pew cushions. The absorption of the pew cushions was also measured on the floor of the reverberation chamber without the pews. The added absorption of pew cushions is influenced by the presence of the pews, is largely independent of the pew sample size, and is predictable from reverberation chamber measurements.

Predicting theater chair absorption from reverberation chamber measurements

The sound absorption of small samples consisting of up to 18 upholstered theater chairs was measured in a reverberation chamber and these results were related to the effect of larger samples of the same chairs in auditoria. The sound absorption coefficients of the chairs, measured in a reverberation chamber, were found to be linearly related to the ratios of perimeter to area of the samples of chairs. Extrapolating these reverberation chamber measurements to smaller perimeter to area ratios, characteristic of seating blocks in auditoria, produced good agreement with measurements of the same seats in the auditoria. This procedure is proposed as a more accurate approach to predicting the absorbing characteristics of upholstered theater chairs in auditoria. Three other more approximate approaches were also considered including the use of screens to eliminate edge absorption, average sample size corrections, and the use of average absorption coefficients for all types of chairs. The merits of each approach are discussed and data are given for average sample size corrections.

Use of “corner microphones” for sound power measurements in a reverberation chamber

A comparison was made between acoustic measurements conducted with microphones mounted in the trihedral corners of the 425‐m3 NBS reverberation chamber and similar measurements using microphones located in the room interior, away from the room boundaries. Measurements of broadband and discrete‐frequency sound pressure and of reverberation time were included. It was found that for frequency bands below the 200‐Hz 1/3‐octave band, the difference in sound pressure level between the corner and interior locations was, in general, more than 9 dB. In addition, the variations in the broadband steady‐state sound pressure level and in the reverberation time were much less among the corners than among the interior locations for frequencies below 100 Hz. It is concluded that for the lower frequency bands, use of the corner locations may reduce systematic errors associated with the interference patterns and provide greater measurement precision because of the lower spatial variance.

Use of ‘‘corner microphones’’ for sound power measurements in a reverberation chamber

A comparison was made between acoustic measurements conducted with microphones mounted in the trihedral corners of the 425‐m3 NBS reverberation chamber and similar measurements using microphones located in the room interior, away from the room boundaries. Measurements of broadband and discrete‐frequency sound pressure and of reverberation time were included. It was found that for frequency bands below the 200‐Hz 1/3‐octave band, the difference in sound pressure level between the corner and interior locations was, in general, more than 9 dB. In addition, the variations in the broadband steady‐state sound pressure level and in the reverberation time were much less among the corners than among the interior locations for frequencies below 100 Hz. It is concluded that for the lower frequency bands, use of the corner locations may reduce systematic errors associated with the interference patterns and provide greater measurement precision because of the lower spatial variance.

Pure‐tone measurements in a reverberation room

Recent experimental results obtained at the National Research Council of Canada confirmed the theoretical prediction that the standard deviation of levels for pure‐tone excitation in a room without moving elements is 5.57 dB. However, contrary to the general belief, the reverberant field created here is not random. Two‐point space‐time cross‐correlation measurements reveal that the field is definitely coherent. This experimental finding is supported by a theoretical analysis based on the modal theory of room acoustics. Some comments and recommendations for the present standards concerning pure‐tone power measurements in reverberation chambers will be given.

Sound power measurements in reverberation chambers

A great deal of effort was devoted in recent years to improve the accuracy of the reverberation chamber method for sound power measurements of small sources. The paper summarizes the results achieved. The accuracy problems exist in two areas. The actual radiation of sound power as related to the power radiated into the free field. This ratio is a function of source position and source size as well as the radiation pattern of the source. Quite recently the statistics of the power radiation for a monopole source has been found (R. Waterhouse). Another area affecting the accuracy of measurements is the determination of the time-space average of pressure squared. For this purpose, various means, primarily the rotating diffusers, have been employed. The recent results on the effect of rotating diffusers on the statistical properties of the sound field is presented. Finally, suggestions for future development are given.

Estimation of monopole power radiated in a reverberation chamber

A monopole source radiates a pure tone. It is desired to estimate the free-field value W0 of its power output from reverberation chamber measurements, in which both source and receiver positions are varied. From the known distribution of values when only the receiver position is varied, the statistics of estimates of W0 are derived.