The Kundt tube has been applied to the measurement of the absorption coefficient of materials. Used in this way it is commonly a standing- wave tube or an impedance tube. No matter what name it bears, it is a very handy device for quickly and accurately determining coefficients. It also has the advantage of small size, modest demands in terms of supporting equipment, and it requires only a small sample. This method is primarily used for porous absorbers because it is not suited to those absorbers that depend on area for their effect such as vibrating panels and large slat absorbers.

The construction and operation of the impedance tube are illustrated in Fig. 9-2. The tube usually has a circular cross section with rigid walls. The sample to be tested is cut to fit snugly into the tube. If the sample is intended to be used while mounted on a solid surface, it is placed in contact with the heavy backing plate. If the material is to be used with a space behind it, it is mounted an appropriate distance from the backing plate.

Figure 9-2: The standing-wave tube method of measuring the absorption coefficient of absorbing materials at normal incidence.

At the other end of the tube is a small loudspeaker with a hole drilled through its magnet to accommodate a long, slender probe tube coupled to a microphone. Energizing the loudspeaker at a given frequency sets up standing waves due to the...