The Determination of Reverberant Sound Absorption Coefficients from Acoustic Impedance Measurements

Abstract

A procedure is outlined whereby acoustic impedance measurements may be utilized to predict reverberant sound absorption coefficients. The average of coefficients for the six standard frequencies so determined agrees closely with the average reverberant coefficient, for cases where the material may be said to obey the normal impedance assumption. A method of utilizing acoustic impedance tube measurements in the field, for acceptance testing, is indicated which should be applicable even in cases where the normal impedance assumption is violated. On the theoretical side, best agreement with experiment was obtained by utilizing a new kind of statistics which associates, with each wave packet in a random field, a scalar quantity equal to the square of the absolute value of the sound pressure in each packet, instead of the customary energy flow treatment. On the other hand, it was found necessary to utilize a semi-empirical concept of equivalent impedance as distinguished from the usual complex impedance treatment.