Noncontact Measurements of Sound Absorption Coefficient with a Pressure-velocity Probe, a Laser Doppler Vibrometer, and a Microphone Array

Kundt's tube and reverberant chamber are common methods for determining the sound absorption coefficient or acoustic impedance of materials. These measurement methodologies are well-known and standardized, albeit not being practicable in-situ and requiring the isolation of samples of the material under test. Furthermore, Kundt’s tube results are affected by the size and the diameter of sample, and by the tube length, while reverberant chamber ones by the room dimensions and diffusiveness. In literature, noncontact techniques for sound absorption coefficient and acoustic impedance measurement are widely debated. In this paper, three different noncontact systems for measuring the sound absorption coefficient have been investigated: a pressure-velocity probe, a Laser Doppler Vibrometer, and a spherical microphone array featuring 64 capsules. The three methods have been evaluated through in-situ measurements of materials with known acoustic characteristics: Basotect G+ and Expanded Polystyrene. Furthermore, the results obtained with the standard test signal, i.e., white noise, are compared with the exponential sine sweep technique, which provides an increased signal to noise ratio, and allows for removing nonlinear high order distortions and acoustic reflections. As a main contribution of this work, it will be shown that microphone arrays are an optimal solution for measuring the sound absorption coefficient.