The acoustic impendance of perforates at medium and high sound pressure levels

A study has been carried out of the behaviour of the acoustic impedance of a range of perforates at medium and high incident sound pressure levels. It included a detailed experimental programme together with a theoretical analysis of the problem. The theoretical work yields, for the linear impedance range, an improved formula for the impedance of perforates which is based on that for a single isolated orifice modified for interaction effects. This prediction technique is substantiated by the results of the experimental programme. Theoretical modelling of the non-linear impedance is based on a quasi-steady approximation to the acoustic flow through the orifice; this approach yields a formula for the non-linear acoustic resistance based on steady flow data for the perforate, which is readily obtainable. Further, the analysis yields a method of predicting when the non-linear mass reactance attains its asymptotic minimum. The correlation between experimental and theoretical results is good. Other associated aspects of the work are discussed in some detail. These include (i) the coupling of the mechanical and acoustic impedance of the perforated plate, (ii) the effects of honeycomb in the backing cavity and (iii) the two-microphone method of impedance measurement. Finally, some consideration is given both to modifying the non-linear behaviour of perforates and to applications of the prediction formula where the pressure rather than the velocity field is specified.