Laser Doppler vibrometer-based acoustical measurement of reflection coefficient up to 20 kHz

Abstract

The well‐established two‐microphone technique for determining the acoustic reflection of a material in an impedance tube has physical limitations that hinder the measurement of this quantity at high frequencies. Tube geometry, microphone spacing, and perturbation of the sound field by measurement instruments make the determination of normal incidence reflection coefficients above 10 kHz difficult, if not impossible, using microphones and the traditional impedance tube methods. In a recent work, an extended impedance tube technique utilizing a scanning laser doppler vibrometer (SLDV) was presented [Vanlanduit et al., J. Acoust. Soc. Am. 3 (2005)]. In this study, a method using a single point laser Doppler vibrometer (LDV) is used to study the limitations and merits of acousto−optic sensing in impedance tubes. Foams of various thicknesses are measured using the transfer function method in a transparent impedance tube. Results are presented that convey the potential usage of these acousto−optic techniques in the measurement of acoustic reflection properties of materials up to 20 kHz and beyond.