Sound absorption coefficient measurements in a reverberation chamber using damping densitya)

The measurement of sound absorption coefficients in a reverberation chamber often involves uncertainties owing to the insufficient diffusivity of the room sound field, which results from the low modal density at lower frequencies. This paper proposes a measurement method that uses damping density (DD) to address this problem. The DD treats the damping constants (DCs) at each frequency as a probability density function, and the DCs at each frequency are calculated from the room impulse response. A preliminary study showed that the proposed method yielded lower reverberation times (RTs) than conventional methods while maintaining measurement stability. Furthermore, the results confirmed that the proposed method successfully evaluated the initial decay characteristics. Measurements of 200 mm-thick urethane foam in an actual reverberation chamber demonstrated that the proposed method yielded intermediate RTs between early decay times and conventional RTs in the low-frequency range (below 315 Hz) under empty room conditions and achieved improved measurement stability across multiple measurement paths. The resulting sound absorption coefficients showed the smallest relative errors compared with the theoretical values in the 80-250 Hz range, except at 200 Hz.