On improving low-frequency sound absorption using a compound microperforated panel backed by a panel-type resonator with tuned multi-frequency resonators

The present work is aimed to broaden the half-absorption bandwidth and improve low frequency sound absorption of a microperforated panel (MPP) by investigating the effects of tuned multi-frequency resonators attached to a flexible panel (i.e. a multi-frequency panel resonator – mMPR), as the backing of MPP so to create a compact sound absorber. The effects of different configurations of multi-frequency resonators were investigated through theoretical and experimental studies. The results demonstrated that the coupling strength among MPP, the panel and resonators at their resonant frequencies had a significant contribution to shaping the sound absorption characteristics, that were particularly influenced by the placement of resonators, quantity of resonators and the configuration of resonators, and these parameters could be tuned to extend the half-absorption bandwidth as well as the low frequency sound absorption coefficient. It was found that there was minimal mass of resonators for them to be effective in improving low frequency sound absorption characteristics and an optimal mass ratio demonstrated a better sound absorption performance in terms of bandwidth and absorption coefficient without an unnecessary increase of structural mass. A circular arrangement of multi-resonators was found to be beneficial in achieving relatively similar MPP-panel-resonator coupling of multi-resonators so to achieve a more uniform sound absorption coefficient within the particular bandwidth. It was experimentally demonstrated that a 200 mm × 200 mm MPP with the backing of a panel and circularly-arranged multi-frequency resonators of only 2 % mass ratio, could achieve a wide low-frequency half-bandwidth from 160 Hz to 550 Hz with the averaged absorption coefficient improved by 18 % to 0.72 over 150 Hz to 260 Hz, in the frequency range within which the absorption valley was previously located.