Acoustic impedance of perforations in contact with absorbing material

Abstract

Acoustic impedance of perforations in contact with fibrous material is experimentally determined for various hole geometries and filling densities in the absence of mean flow. Two setups are developed to measure the perforation impedance and the acoustic properties of the absorbent, while yielding empirical expressions for both. The experimental results show that both real and imaginary components of the perforation impedance substantially increase as a result of the contact with the fibrous material, whereas they generally decrease with increasing porosity for a given filling density. The empirical expressions developed in this study are implemented in a boundary element method to predict the transmission loss of dissipative silencers, which is then compared with the experimental results.