Comparison of different methods for surface impedance and transmission loss measurements in duct

摘要

Impedance tube is am important tool to measure surface impedance of material, for which two methods have been standardized. Recently, people also try to use an "extended" impedance tube to estimate sound transmission loss because of the simplicity. A few manufacturers have even included this feature in their products. With the extension tube after the test sample ("downstream" as it is often called), people may also adopt the transfer-matrix method (or equivalent "scattering-matrix" method) used in an acoustical two-port system to measure reflection and transmission coefficients, and hence the surface impedance and the transmission loss of the test sample in the condition of an anechoic termination. Those methods together with the standard impedance tube method are compared experimentally in this paper for different test samples. Same results are obtained only for a certain given conditions. This is because that the physical configurations for the extended impedance tube and the standard impedance tube, as well as the two-room method for the sound transmission loss problem, are different. One has to be careful to check if the situation is the same as that in the assumption when applies measured results to practical cases.