In-duct measurement of acoustic properties using a two-microphone technique with random and pure tone excitation

Abstract

A two-microphone technique has been used to determine acoustic properties of materials using pure tone and random excitation. Theory shows that the acoustic properties may be derived from magnitude and phase angle measurements for a pair of microphones. An alternative derivation based on four-pole theory will also be presented. Absorption measurements of glasswool samples using pure tones show discrepancies when compared with the conventional standing-wave method. These errors are traced to phase angle measurement uncertainty. It will be shown that the use of random excitation avoids these errors, and that absorption measurements using random excitation compare well with measurements using the standing-wave method. However, there are bias and random errors generated during signal processing of random data. These errors may be minimized by proper selection of signal processing parameters.