PREDICTION AND MEASUREMENTS OF THE INFLUENCE OF BOUNDARY-CONDITIONS IN A STANDING-WAVE TUBE

The paper concerns a comparison between measurement results of the aco ustic absorption coefficient and impedance using two different methods , the standing wave tube (Kundt's tube) and a free field method. The m ounting of the sample in the Kundt's tube is known to be very importan t. In this paper, the effect of the constraints at the tube wall on th e absorption coefficient of an elastic porous material is analysed. It will be shown that the effect of the friction offered by the tube wal l is to stiffen the material, compared to its behaviour in the foe fie ld. Measurement results together with model calculations are presented . The material parameters used in the models have been determined by s eparate measurements.