Acoustic impedance, absorption and reflection curves are measured with the Kundt's tube. This tube consists of a cavity with the specimen to measured placed at one end. At the other end a loudspeaker is located, which emits a plane wavefront of the same sound pressure level at all frequencies.
There are two microphones that allow calculation of the transfer function between the incident wave and the reflected wave. In this way the three curves (acoustic impedance, absorption and reflection) are obtained.