Estimative of the errors caused by the sample size effect on the in situ measurement of the absorption coefficient

摘要

The Boundary Element Method (BEM) is used in the prediction of the sample size effect onrnin situ impedance measurements. The BEM model simulates the measurement of thernacoustic impedance above the sample such as in a measurement with a combined pressurernand particle velocity sensor. A locally reactive sample is simulated, with a point sourcernabove the surface at normal incidence. As the sound source is placed close to the sensor thernwave fronts can not be considered plane. Because of that, two methods to obtain the surfacernimpedance, based on the acoustic field description, are presented and compared. It is alsornshown that the sample size, the thickness and resistivity of the sample, and the position ofrnthe sound source cause different errors in the calculated surface impedance and absorptionrncoefficient. Those effects are parameterized in a way that the user of the in siturnmeasurement setup may have an estimative of the required sample size for a desiredrnfrequency range with a given error allowance.