An acoustic impedance measurement of a parallel-pore stack used to determine the thermoviscous functions of the stack

Abstract

Recently, thermoviscous functions have been measured for single pores of various cross-sectional geometry [L. A. Wilen, J. Acoust. Soc. Am. 103, 1406–1412 (1998)]. This investigation consists of measuring the acoustic impedance through a stack with parallel pores, inverting to yield the thermoviscous functions F(λ) and F(λT), and determining the accuracy with which the inversion can be accomplished. The measurement shall be performed using a standard impedance tube with the typical measurements taken, i.e., pressure maximum, pressure minimum, and the positions of two pressure minima. In order to be able to solve for the thermoviscous functions, a second measurement must be taken with the stack positioned away from the rigid termination, leaving a region of air. The impedance translation theorem (modified to account for the stack porosity) will have a functional form containing both F(λ) and F(λT) and shall be employed for both stack placements to provide two equations which can be solved for the two unknowns, F(λ) and F(λT), for a given frequency. If the method proves successful, modifications shall be made to the experimental setup so that it can accommodate a temperature gradient imposed across the stack. [Work supported by ONR.]