Jonathan H. Alexander

Transfer impedance of microperforated materials with tapered holes

It has been previously shown that relatively simple computational fluid dynamics (CFD) models can be used to calculate the transfer impedances, including the associated end corrections, of microperforated panels. The impedance is estimated by first calculating the pressure difference across a single hole when a transient input velocity is imposed, and then Fourier transforming the result to obtain the impedance as a function of frequency. Since the size of the hole and the dimensions of the inlet and outlet channels are very small compared to a wavelength, the flow through the hole can be modeled as incompressible. By using those procedures, Bolton and Kim extended Maas classical theory to include a resistive end correction for sharp-edged cylindrical holes which differs from those previously proposed by the inclusion of a static component. Here it is shown that CFD models can also be used to compute end corrections for tapered holes. Since practical experimental characterization of perforated materials often involves measurement of the static flow resistance, a closed form empirical equation for that quantity has been developed. Finally, it is shown that configurations having equivalent static flow resistances can yield different acoustic absorptions.

Effect of lining anisotropy on sound attenuation in lined ducts

In this paper, the effect of duct‐lining anisotropy, with respect to flow resistivity, in particular, is considered. The duct lining is modeled by using a fully poroelastic theory derived from the Biot theory. The lining is considered to occupy only a part of the duct cross section, and is imagined to be infinite in length. When the appropriate solutions for the sound fields within the airway and the anisotropic porous material are substituted into the boundary conditions of the problem, a homogeneous system of equations results. The propagation characteristics of the duct modes can then be found by identifying the conditions under which the determinant of that system of equations is zero. Corresponding experiments have been conducted using samples of aviation‐grade glass fiber partially lining a square‐section, four‐microphone standing wave tube. Excellent agreement between measured attenuations and predictions was found. It will be shown that the modal attenuation is primarily sensitive to the flow resi...