Behavioral criterion quantifying the effects of circumferential air gaps on porous materials in the standing wave tube

The influence of mounting conditions on the measurement of the absorption coefficient in the standing wave tube (swt.) is investigated. In a previous paper [D. Pilon, R. Panneton, and F Sgard, J. Acoust. Soc. Am. 114, 1980-1987 (2003)], a behavioral criterion was developed to access the effect of a circumferential edge constraint on elastic foams. Similarly, the effect of circumferential air gaps on porous materials is studied here. The objective is to identify the materials, in terms of a ratio based on their physical properties, for which it is possible to measure the theoretical absorption coefficient when mounted inside the swt with a circumferential air gap. The difference between the measured and theoretical absorption coefficient is evaluated for a wide range of materials with various sample sizes. These errors are then sorted in terms of the chosen ratio. It is shown that for certain values of the ratio, the theoretical absorption coefficient can be efficiently measured using the swt. Through the use of this acousto-visco-inertial criterion, an experimenter can determine which absorption is going to be measured: the theoretical absorption or one that will be influenced by the circumferential air gaps. (C) 2004 Acoustical Society of America.