Abstract: The experiments are based on the detection of air-borne slow compressional waves after transmission through absorbent porous materials. The measurement of the attenuation and the velocity of the slow wave at ultrasonic frequencies enables us to determine some important material parameters which are needed for the prediction of the acoustic performance (in the audible range) of the samples. In addition, an XY positioning system is useful for probing the samples at different positions. The difference in velocity and attenuation as a function of the measuring point on the sample gives information on the homogeneity of the material. This leads to non-destructive testing and to a study of the difference in material properties within one sample.