PARALLEL-ELEMENT LINER IMPEDANCES FOR IMPROVED ABSORPTION OF BROAD-BAND SOUND IN DUCTS

Zwikker and Kosten's theory for propagation of sound in channels was u sed to predict surface impedance and the spectra of sound absorption f or parallel-element acoustical liners, Liner configurations ranged fro m lumped-element structures with thin acoustically resistive face shee ts to ceramic honeycomb structures for which the sub-millimetre-diamet er channels provided distributed resistance. Excellent agreement was o bserved between measured and predicted normal-incidence impedances for constant-depth and variable-depth ceramic-honeycomb structures, For s ingle-degree-of-freedom, lumped-element absorbers, the experimentally observed increase in resistance at frequencies near anti-resonances wa s predicted, It has been unnecessary to include viscous dissipation ne ar the cavity walls to adequately model the impedance well away from a nti-resonance. Inclusion of cavity-wall dissipation allows prediction of the increased resistance at frequencies near anti-resonance. Predic tion of this behavior in the region of anti-resonance frequencies may be a critical consideration if parallel elements are to be exploited f or increased attenuation bandwidth. (C) 1995 Institute of Noise Contro l Engineering.