The attentuation of dissipative flow duct mufflers with internal mean fluid flow in the absorbent

Abstract

In dissipative flow duct mufflers incorporating layers of porous bulk sound absorbent, mean flow within the pores of the material is inevitable because the porous medium is exposed to pressure gradients in the duct flow. The acoustical effects of internal mean flow can be considerable, particularly in the case of internal cumbustion engine exhaust mufflers of the dissipative type. Theoretical treatments are presented here, both to describe the effect of internal flow on the bulk acoustic properties of the porous medium and to find the effect of the absorbent in situ, in the form of the sound transmission loss of the silencer. The measured transmission loss of an experimental silencer is compared to predicted data, and good agreement between the two is obtained. The effects of mean fluid flow in the central passage and internal flow in the absorbent are separately demonstrated.