Measuring sound absorption using local field assumptions

Abstract

To more effectively apply acoustically absorbing materials, it is desirable to measure angle-dependent sound absorption coefficients, preferably in situ. Existing measurement methods are based on an overall model of the acoustic field in front of the absorber, and are therefore sensitive to deviations in the actual setup from the assumed measurement setup. In order not to be restricted to ideal measurement setups only, two novel methods are developed with the research described in this thesis. These methods, the Local Plane Wave (LPW) method and the Local Specular Plane Wave (LSPW) method, are both based on a local field decomposition. It is assumed that the acoustic field can be approximated locally with a combination of one incident and one reflected wave. The LSPW-method encompasses the LPW-method, and is therefore the most universal variant. This method requires measurement of two acoustic pressures or measurement of acoustic pressure and particle velocity in the surface normal direction to determine the angle-dependent sound absorption coefficient. The effect of area-averaging is investigated both numerically and experimentally. The results show that area-averaging is effective in reducing undesirable effects, as for instance caused by reflections from the environment of the setup. In combination with area-averaging, the LPW- and LSPW-method have potential for application in situ. In addition, as many kinds of absorbing surfaces have an inhomogeneous structure or material, an area-averaged sound absorption coefficient is a more appropriate indicator than a point-based coefficient. A very welcome spin-off from the present research is the development of a novel type of 3D sound intensity probe. The application of 8 small MEMS-microphones, and the chosen placement thereof, allow increased accuracy of sound absorption measurements that are performed with the LPW- or LSPW-method, in particular for poorly absorbing surfaces. In addition, a novel free-field probe calibration method is presented. An advantage of this method is that the directivity characteristics of the sound source do not need to be known a priori.