The techniques frequently used in laboratories to measure the absorption coefficient of materials tend to be quite tedious or too sophisticated for industrial purposes. Indeed, quite often, only comparative control of the acoustical properties of a material, essentially in the low frequency domain (up to 1500 Hz) is needed. A method is presented in this paper which rapidly estimates the absorption coefficient and the relative quality of different samples of the same material at certain predetermined frequencies. This method makes use of a compaet apparatus, containing a 'sensor', which provides the exciting signal, the acoustic pressure and the acoustic velocity near the material in a cavity, as well as suitable electronic equipment. Experimental results are presented which show that the technique is both reliable and sufficiently accurate.
INTRODUCTI ON
A good knowledge of the acoustical properties of materials is frequently necessary. Certain well known laboratory techniques are needed to measure the properties. However, such techniques are not really convenient for the permanent control of products manufactured in factories because they are tedious and/or too sophisticated. On the other hand, the acoustical control of materials does not really require very precise measurement of their properties but only good reproducibility of 257