Variations in Sound Absorption Coefficients as Obtained by the Reverberation Chamber Method

Abstract

HE characteristic of an acoustical material which is of greatest interest is usually its sound absorbing efficiency. The efficiency is expressed numerically as an absorption coefficient, which may be defined as unity minus the ratio of sound energy reflected to the energy impinging on its surface. A study of published coefficients on samples of the same material measured by different laboratories shows considerable variation in the numerical values, particularly in the case of materials having coefficients greater than 50 percent. While some variation is tolerable, it is essential for purposes of calculation that the numerical values be in reasonably good agreement to permit a fair comparison between different materials. Sound absorption coefficients are most commonly measured by the reverberation chamber method, in which the efficiency of the sample is determined indirectly by its effect on the reverberation time of a room. Since the widest use for sound absorption coefficients is in the calculation of reverberation times for rooms and auditoria, this method facilitates the practical application of the coefficient obtained. The technique of reverberation time measurement by electrical methods employed by the various laboratories is similar and involves the measurement of the rate of decay of the sound energy over a range of 30 or more decibels. When reverberation time determinations were made by ear, a large chamber was desirable in order that the reverberation be considerable, even when the total sample was present. However, the improvement and development of electrical and electronic equipment has made possible the accurate measurement of time periods in the order of tenths of a second. Reliable measurements of reverberation time can therefore now be made in a relatively small chamber.Recently published data • on the coefficients of the same sample tested at various laboratories indicated that there was a tendency for the coefficient to decrease as the volume of the test room was smaller. The authors therefore felt that a critical examination of the discrepancies which occurred between the results obtained in the Johns-Manville Laboratories and the National Broadcasting Company Laboratories might be productive, since these two test chambers were widely different in cubical contents. Table I shows a series of values on five different materials tested in two laboratories. The last two tests are on different samples, but in both cases the material is subject to regular factory control tests and hence is not likely to vary appreciably.It will be noted that in some cases considerable discrepancies exist between National Broadcasting Company and Johns-Manville coefficients. These discrepancies are more pronounced when the efficiency of the material is high. The reverberation chamber employed by the TABLE I. Absorption coefficients. Sabine formula.FREQUENCY Material A Material B Material C Material D Material E (Mounted on Furring) Laboratory J.M. NBC J.m. NBC J.M. NBC J.M. NBC J.M. NBC Absorption coefficients expres...

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Effects of sound radiation on Sabine absorption coefficient of modally-reactive panels

“…So, in numerous experiments where the absorption coefficient of modallyreactive structures was measured [11][12][13], the sound radiation from the structures was inherited in the measured results. Also, it is known that the absorption coefficient is a parameter which depends on the conditions of the sound field in the room where the measurement is conducted [11,14,15]. This means that the sound absorption and radiation and thus, the absorption coefficient of the same absorptive surface, are functions of the enclosed sound field and they are different in different rooms.…”