Comparison of Reverberation-Room and Impedance Tube Measurements on Carpets

Abstract

Carpet is a material difficult to define in terms that relate to its sound absorbing capacity. Accordingly there is use for a simple method of measuring its sound absorption. The reverberation room method (ASTM C423) provides the definitive measure for random incidence absorption when large specimens are available. The impedance tube method requires only small specimens but measures normal-incidence absorption, which is not a useful measure of performance in rooms. For some materials, deemed “locally reacting” with respect to the incident sound, one may calculate random-incidence absorption from tube data, but this cannot be assumed, especially for a loose ensemble of components such as carpet. Accordingly the correspondence between the two methods of measurement was studied for a range of carpet specimens. Results indicate that calculations from impedance tube data, corrected according to the locally reacting assumption and taking account of diffraction effects in the reverberation room, yield lower absorption coefficients than are obtained directly in the reverberation room. Since a good correspondence is found with truly locally reacting materials, it is inferred that carpet does not thus qualify.