Microphone-free measurement of acoustic absorption coefficient of materials using a standing wave tube

Abstract

This paper reports a method for measuring the specific acoustic impedance or absorption coefficient of acoustic material using a standing wave tube without a microphone. The method was validated by tests conducted in a normal standing wave or impedance tube—a straight solid-wall tube with a speaker and a material specimen perpendicularly mounted at each end, respectively. Assuming that sound waves are planar in the tube, we derived the relationship between the impedance at the input and output ports of the loudspeaker as well as the same for the standing wave tube. Through these two relationships, the specific acoustic impedance of the material specimen is then expressed as a function of the input electrical impedance of the loudspeaker together with a few parameters characterizing the electro-acoustic system. With the resulted formula, probing the input voltage and current to the speaker alone allows quantitative evaluation of the specific acoustic impedance of the material specimen. Experimental results show that the method can correctly identify the absorption coefficient of acoustic materials in the lower frequency range. The results are less accurate in the higher frequency range due to the invalidity of the assumption of plane waves and the low signal-to-noise ratio. From the results, it is also observed that, due to the microphone-free feature, the reported method is suitable for in situ study of acoustical or vibrational conditions of the material or structural specimen at the end of a duct with geometry making microphone installation difficult or impossible.