SOUND-ABSORPTION BY PSEUDOSTOCHASTIC DIFFUSERS (SCHROEDER DIFFUSERS)

This paper attempts to explain the unexpectedly high sound absorption at low frequencies, found by Fujiwara and Miyajima (Absorption characteristics of a practically constructed Schroeder diffuser of quadratic-residue type; Appl. Acoust., 35 (1992) 149 52) with pseudostochastic diffusers (so-called 'Schroeder diffusers'). For this purpose, a simple model is employed, assuming the total sound pressure in the diffuser plane to be constant, which is considered a reasonable approximation in the low frequency range. Consequently, strong equalizing flows develop between the elements of the diffuser, leading to much higher energy losses than those effected by channels of equal depth. A more rigorous treatment of the absorption by calculating the first Hartree harmonics shows that the assumptions of the simple model are justified in the low frequency range. The efficiency of the diffusers as low frequency absorbers is demonstrated by various examples.