W. T. Chu
Acoust. Lab., Inst. for Res. in Construction, Natl. Res. Council, Ottawa, ON K1A OR6, Canada
It has long been recognized that the acoustic impedance is the chief characteristic of the performance of acoustical materials. The impedance tube provides an easy way of measuring this characteristic at normal incidence. It is also known that the effect of tube attenuation must be included in the impedance tube analysis for accurate measurements. Although theoretical prediction can be deduced from the well-known Helmholtz--Kirchhoff formulation, disagreements with experimental results (up to 15%) have been reported. Previous results relied on locating and measuring the pressures as a number of maxima and minima along the standing wave pattern inside the tube. The method was tedious. A more rapid method for measuring the tube attenuation is possible based on the recently proposed three microphone method for impedance tube measurements [W. T. Chu, J. Acoust. Soc. Am. 92, 2315(A) (1992)]. The procedure will be presented together with results from both computer simulations and actual experiments.