Measurement method with a pressure-velocity sensor for measuring surface normal impedance of materials using ensemble averaging: Comparison with other methods and its geometrical configuration

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In an earlier paper [T. Otsuru et al., J. Acoust. Soc. Am., 125, 3784–3791 (2009)], the theoretical development and concept of “ensemble averaged” surface normal impedance were summarized by some of the authors using the math–physical model based on the boundary element method (BEM). This paper elaborates further on past discussions of the measuring method of the surface impedance of materials. First, three materials, namely, 50-mm-thick glass wool, 25-mm-thick glass wool, and 10-mm-thick needle felt, were measured in a reverberation room to compare the absorption characteristics measured using sensors of different types. Measured data show good agreement between the sensor types in the absorption coefficients, while some discrepancies are seen in the impedances. Next, the feasibility of our method in terms of sound absorption characteristics was confirmed using a series of measurements in comparison with the impedance tube method. Finally, the effects of sample size and receiver-to-sample distance are presented both in simulation and measurement for investigating the level of utility in various applications. The resulting absorption characteristics are examined to elucidate an appropriate measurement setting and demonstrate the general utility of the method.