Mutual coupling of resonators: The effect of excessive acoustic resistance and its reduction by a local acoustic resistance

Hybrid resonance structures, characterized by parallel arrangements, play a crucial role in expanding the bandwidth of sound absorption. It involves the assembly of a set of resonators with different resonance frequencies to form broadband sound absorption a peak-dip band structure. While the mechanism behind peak formation is well elucidated, the genesis of dips remains shrouded. In this study, we utilize two coupled resonators to demonstrate that dips arise from anti-resonance, induced by the mutual coupling of resonator impedances with neighboring resonance frequencies. Further investigations uncover that the origin of mutual coupling can be attributed to the velocity dipole response of coupled resonator, resulting in the generation of evanescent waves and the effect of excessive acoustic resistance. Contrary to intuition, we introduce local acoustic resistance by shunt electromechanical diaphragm (SEMD) to mitigate this excessive acoustic resistance of the coupled resonators, exhibiting a promising reduction in overall acoustic resistance at the anti-resonance frequency. Subsequent exploration reveals that this 'acoustic resistance reducing acoustic resistance' approach significantly enhances sound absorption at the dip frequency, particularly under random incidence conditions. Our research encompasses analytical, numerical, and experimental studies, which are in concordance with each other.