Prediction of random incidence sound absorption coefficients of porous materials

• Five methods are used to determine the acoustic properties of porous materials. • Random SACs predicted using FTMM with the geometric or maximum incident power. • Comparison of random SACs measured and predicted in small and large rooms. • The characteristics of each method are discussed, and recommendations are made. • The measured and inverse method flow resistivities are compared. Measuring the random incidence sound absorption coefficient (SAC) of porous materials is costly and requires a full-size reverberation room. This paper shows that the predictions from impedance tube measurement and the empirical method have reasonable accuracy for the determination of the random incidence SAC above 400 Hz. It also shows that the measurement of the random incidence SAC in a small self-made reverberation room is consistent with the full-size reverberation room results in the frequency range above 800 Hz. The prediction of the random incidence SAC was made using the finite transfer matrix method (FTMM) with material data obtained using the two-cavity method, the two-thickness method, the four-microphone transfer function matrix method, the empirical equations, and calculation by applying the inverse method to the Johnson-Champoux-Allard (JCA) model. In general, the different methods show a similar trend and reasonable agreement in the middle and the high-frequency ranges when predicting the acoustic properties of fibrous materials. The characteristics of each method are compared and discussed, and recommendations for the use of each method are made. The ‘usual SAC’ method, which uses the geometric incident acoustic power, agrees with the full-size reverberation room results better, while the ‘true SAC’ method, which uses the power absorbed by an absorber whose impedance is equal to the radiation impedance of an infinite size absorber as the incident acoustic power, is more suitable for predicting the results obtained in the small-size reverberation room.