Effect of absorber geometry on apparent absorption coefficients as measured in a reverberation chamber

Measurements were made in the NBS reverberation chamber to determine the apparent random incidence absorption coefficient as a function of the area, perimeter, and shape of the test specimens for three different materials arrayed as combinations of rectangular pieces. Measurements made with the specimen edges exposed and with the edges covered with reflecting material were compared. The experimental results were compared with a theoretical model [T.D. Northwood, J. Acoust. Soc. Am. 35, 1173–1177 (1963)] using values of the acoustical admittance obtained from impedance tube measurements performed on the same materials. Both the experimental and theoretical results indicate that the random incidence absorption coefficient increases approximately linearly with E, the ratio of the perimeter to the area of the specimen for values of E ranging from 1.3 to 3.3 m−1. The theoretical analysis indicates, however, that a linear extrapolation to the value E = 0 is not necessarily valid. It was also found that if the specimen area and perimeter are kept constant, the values of the random incidence absorption coefficient decrease by a few percent as the specimen shape is changed to increase the number of vertices (inside plus outside right-angle corners) in the configuration.