Page 245 - Master Handbook of Acoustics

FIGURE 12-2 The impedance 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 interaction of the outgoing wave with the wave

reflected from the sample. The form of this standing wave gives important information on the
absorption of the material under test.
The sound pressure is maximal at the surface of the sample. As the microphone probe tube is
moved away from the sample, the sound pressure falls to the first minimum. Successive, alternating
maxima and minima will be detected as the probe tube is further withdrawn. If n is the ratio of the

maximum sound pressure to its adjacent minimum, the normal (perpendicular) absorption coefficient
α is equal to:
n

This is plotted in Fig. 12-3.