Error estimation due to sample size effects of in situ surface impedance measurements.

Abstract

The boundary element method (BEM) is used to predict the effect of the sample size on in situ impedance measurements on a locally reactive sample at normal incidence. The BEM model is based on a direct measurement of the acoustic impedance above the sample (with a combined sound pressure and particle velocity sensor). As the sound source is placed close to the sensor, the acoustic waves cannot be considered plane anymore. Because of that, two models to obtain the surface impedance based on the acoustic field are presented and compared. It is also shown that the sample size, the thickness of the sample, and the positions of the source and receiver affect the measured surface impedance and the calculated absorption coefficient. Those effects are parametrized in a way that the user of the in situ measurement setup may have an estimate of the required sample size for a desired frequency range with a given error allowance.