Model for the Acoustic Impedance of a Perforated Plate Liner with Multiple Frequency Excitation

Abstract

The one-dimensional, time-varying equations of momentum and continuity describing the fluid mechanics in a perforated plate liner are integrated numerically in time to obtain instantaneous values of the orifice velocity and displacement. An arbitrary orifice pressure can be used to excite the system. In this way spectral noise excitation can be simulated. This approach is of value since previous experiments have shown that excitation at one frequency can effect the nonlinear acoustic impedances at other frequencies. A quasi-steady nonlinear resistance term was added to the differential equations to account for the effects of orifice velocity and displacement as well as grazing flow. This term was obtained from steady-flow experiments but was assumed to be valid instantaneously. This assumption was at least partially justified by excellent agreement with single-frequency acoustic-impedance measurements over a large range of sound pressure levels without grazing flow. A comparison of the theory with experimental results will be shown. These will include two frequency tests (in which the pressure amplitude at one frequency is varied) and single-frequency impedance measurements in the presence of grazing flow. The effects of some typical noise spectra on liner impedance will also be shown.