Modified impedance tube measurements and energy dissipation inside absorptive materials

Impedance tube is an important tool to measure acoustic properties of a material, for which two methods have been standardized. The methods for a “modified” impedance tube have been developed recently to measure material properties in the condition of an anechoic termination. Instead of commonly used two-load or two-source methods, a method with direct and inverted configuration in cooperation with the single-microphone method is suggested in this paper. With the help of transfer matrix method, combined effect of several materials placed in series can be predicted from individual properties. A simple recurrence formula is obtained to relate the acoustic properties of a multilayer structure with the reflection and transmission coefficients of each layer. Very good agreements are obtained between the prediction and the direct measurements. Energy dissipations inside porous materials for different situations are studied and compared.