Acoustic performance of innovative materials: low-frequency behaviour of reverberation room for ISO-354 measurements

Acoustic metamaterials (AMMs) have been increasingly used in automotive applications due to their excellent acoustic performances and easily customization. The technical procedure employed to characterize the sound absorption coefficient of such materials involves the use of a 10m2 sample according to the ISO-354 standard inside laboratory spaces named rever- beration rooms. However, this type of measurements have some limitations related to the lack of sound field diffusion at low frequency, providing negative contributions to the absorption coefficient. For this reason, the goal of this study is to test a method to investigate the modal behaviour in such specific rooms and the way it affects the outcomes in terms of sound absorp- tion coefficients. With this purpose, a large number of acoustic measurements were carried out in two different reverberation rooms. Then, the experimental outcomes have been compared with the acoustic simulations of the two halls by means of finite-element numerical models. Moreover, a statistical study performed on the modal identification results allowed assessing the relationship between the modal decay times of the most prominent resonance modes and the measured reverberation time in third-octave bands below 200 Hz. Therefore, this work show how the most prominent modal effects negatively affects the sound absorption measure- ments. In other words, this method can be a first step forward to design a scaled reverberation room geometry to test also smaller samples of innovative materials, smoothing the measure- ments procedure and reducing the production costs.