Low frequency sound absorption by compliant Helmholtz resonators

A traditional Helmholtz resonator is a rigid-walled cavity and an open neck, which result in one acoustic resonance. In this study, a soft-walled cavity with an open neck, a compliant Helmholtz resonator, is investigated for broadband and large absorption of sound at low frequencies. By applying the soft materials, the dynamics of the compliant walls and the material impedance are coupled to the acoustic pressure filed in the cavity. Thus the impedance of the compliant resonator system as seen by an incident acoustic wave is determined by the interaction of the wall dynamics, material properties, and acoustic domain. Compared to a traditional Helmholtz resonator with the same geometric dimensions, the compliant Helmholtz resonator exhibits multiple resonances with large absorption of sound. In addition, strategic design of the structural composition and the material selection leads to resonant behavior at frequencies lower than the conventional Helmholtz resonance. Through exploring an analytical model developed to characterize the resonator, the threshold on material and structural compliance to yield traditional Helmholtz behavior is also illuminated.