On the modification of Delany and Bazley fomulae

Abstract

It is well known that the formulae developed by Delany and Bazley for characterising the acoustic properties of porous materials should not be extended to low frequencies. Accordingly, a low frequency correction is required when finding the surface impedance or absorption coefficient of a porous material. This article reviews corrections of a type originally proposed by Miki [J. Acoust. Soc. Japan, 1990;11:19-24] and concludes that there is no physical basis behind these methods and that they should not be used. Instead, a number of alternative methods are available that maintain the correct limiting behaviour for the fundamental properties of a porous material at low frequencies, and two of these methods are briefly reviewed here.