Acoustic Impedance Tube - Two-microphone method

Abstract

Absorption of sound by acoustic materials is a critical issue for acoustical engineering. Applications included sound isolation, improving acoustical quality of room and buildingdesign. Knowing the acoustic properties of materials is important for designing such applications. The normal incidence sound absorption coefficient and other acoustic propertiesof materials can be found out by experiment with an impedance tube.The traveling microphone method is reliable for determining the sound absorption coefficient, however, this method is time consuming and also limited with single frequency operations. The two-microphone method is approximately 40 times faster than the traveling microphone method, but this technique requires more accurate instrumentation setup and experimentation. The setup and experiment procedures of the two-microphone method had discussed in this paper. Amplitude and phase mismatch between microphones can be a source of error. The microphone spacing was also found to be critical, accuracy of results decreased with large microphone spacing at high frequencies. An optimum 50mm microphone separation is suggested for multi-frequency signal range from 125Hz to 2000Hz.The experiment results from the two-microphone method had been compared with results from the traveling microphone method. Results were closed to each other within an interval of +/-0.10. For precision measurements, it is still recommended using the traveling microphone method. However, the two-microphone showed a very high potential can be as accuracy as the traveling microphone method. Suggestions had been given for improving the experiment accuracy.