Some Notes on the Measurement of Acoustic Impedance

A modified form of an earlier design of impedance tube is described here. It is capable of measuring the normal impedance of a sample by the variable length, variable frequency, or traveling microphone methods without disturbing the sample in transferring from one test method to another. The tube also may be used to measure the transmission constant with the aid of a long probe tube. Modern graphical aids to the calculation of the impedance from the quantities measured in the tube are described. Comparison of data obtained by the variable length and traveling microphone methods is made, and good agreement is found over most of the frequency range. It is shown that the clamping effects of the tube walls on the edge of a lightweight, non-rigid sample are serious enough to introduce large gyrations of the impedance curves at frequencies below 500 c.p.s.