A METHOD OF MEASURING THE ACOUSTIC ABSORPTION COEFFICIENT OF A MATERIAL SPECIMEN AT ONE END OF A TUBE USING A DYNAMIC MICROPHONE AT THE OTHER END

This paper reports the development of a method for measuring the absorption coefficient of a material specimen mounted at one end of a planar wave tube using a dynamic microphone at the other end. In the proposed method, the dynamic microphone mounted is used as an actuator (loudspeaker) to generate sound waves and simultaneously performs as a probe to sense acoustic impedance at the same point. For the electro-mechanical acoustical system formed by the dynamic microphone and the tube, a "transduction matrix" is introduced to relate the input electrical variables (voltage and current) and the output acoustical variables (pressure and particle velocity). Measurements of fully-reflected end, anechoic end and a porous material specimen are carried out and compared to the results obtained by the conventional transfer function method. It is found that the results match well with each other in a frequency depending on the length of the tube. In addition to the obvious medical application-in-situ and non-invasive detecting health conditions of eardrum, the method can be utilized in industrial applications where simple and portable apparatus are needed to characterize acoustic materials in-situ and on site.