Measurement of the absorption coefficients of materials and the effect of air spaces between sample and reflector using the two-microphone method

Abstract

The measurement of the absorption coefficient of materials using the two-microphone method is reported. A comparison between the standing wave ratio (swr) and the two-microphone methods using the Celotex tile is made. The results show high correlation in the lower frequency region, but, in the higher frequency region, the result of the two-microphone method is relatively higher than what was measured using the swr. The effect of introducing an air space or cavity between the sample and the hard reflector was also investigated using the 15-mm Celotex acoustic tile and 32-mm polyurethane foam. The results indicated an increase and shifting of the absorption peak towards the lower frequency region. However, it also showed a relative decrease in the absorption of the higher frequencies. As the cavity lengths were further increased, another mechanism was observed leading to the emergence of narrow-band absorption in the higher frequency region which is verified theoretically as associated to the additional sound absorption by cavity resonance.