The paper concerns a comparison between measurement results of the acoustic absorption coefficient and impedance using two different methods, the standing wave tube (Kundt's tube) and a free field method. The mounting of the sample in the Kundt's tube is known to be very important. In this paper, the effect of the constraints at the tube wall on the absorption coefficient of an elastic porous material is analysed. It will be shown that the effect of the friction offered by the tube wall is to stiffen the material, compared to its behaviour in the foe field. Measurement results together with model calculations are presented. The material parameters used in the models have been determined by separate measurements.