Correlation of Absorption Coefficient Values of Acoustical Materials in the Laboratary and Studio

Abstract

The absorption coefficient of an acoustical material has been widely employed by engineers and architects in the acoustical design of enclosures ever since it was first defined by Sabine, and later modified by Eyring and Millington. Unfortunately, however, the values obtained for this parameter in the different laboratories vary greatly due to differences in the methods employed for this purpose. At least two are in great vogue: the reverberation chamber method and the impedance tube method. The former yields values which are invariably higher than those realized in practice, while the values obtained by the latter method do not agree satisfactorily with field values. A statistical coefficient has also been evaluated from the normal incidence tube value, but even this figure does not provide a satisfactory correlation. The reasons for these discrepancies are discussed in detail and it is concluded that values obtained in a semi-reverberant enclosure at random incidence of sound waves are likely to yield values close to those obtained in practice. Such a Test Chamber, set up in the Research Department of All India Radio, is described and results obtained on some typical materials are compared with those obtained by other methods and in the field. It is seen that except at low frequencies, where the resonant modes of the enclosure distort the sound field, the agreement between the test chamber values and field coefficients is better than by other laboratory methods. Finally, the acoustical performance of some typical studios with composite acoustical treatment is analysed on the basis of these results.