Characterization of acoustic materials using the scanning laser Doppler vibrometer
Faculty of Applied Engineering Sciences
Engineering sciences. Technology
7th International Conference on Vibration Measurements by Laser, Techniques, June 19-22, 2006, Ancona, Italy
In this article we will present a method to estimate sound absorption coefficients from measurements of the incident and reflected sound fields near the material under test. The sound fields are visualized using a scanning laser Doppler vibrometer (SLDV). By aiming the SLDV at a rigid (non-vibrating) object and letting the beam pass through a sound field the spatial pressure distribution can be made visible. By visualizing both incident and reflected sound field with respect to a material sample in this manner, the acoustic absorption coefficients can be determined (this is the ratio of the absorbed energy and incident energy). Two alternative set-ups are proposed in this article: a one-dimensional set-up where the sound field (i.e. the standing waves) inside a thin glass tube is measured, and a two-dimensional set-up where the propagating and reflecting sound field between two parallel glass plates is visualized. While the former can only be used to obtain normal incidence absorption coefficients the latter can also be used to estimate oblique incidence absorption values. It is shown that the method is accurate at high frequencies where traditional standardized acoustic material characterization techniques mostly fail.