An ultrathin absorber for broadband low-frequency sound enabled by an impedance boundary

Abstract

The rapid development of acoustic metamaterials has opened new possibilities for creating perfect absorbers; however, achieving compact devices for extremely low-frequency sound remains a challenge. In this work, we present a proposal for the development of extremely low-frequency sound absorbers through coupling dissipated meta-atoms to an impedance boundary (IB). The IB is engineered by a dual-port opening MPP–cavity–MPP sandwich structure, exhibiting sound impedance significantly lower than that of a rigid wall, as well as considerable sound absorption across the entire audible spectrum. To further enhance absorption, a hybridized system integrating over-damped resonators with the IB is proposed, giving rise to a composite impedance boundary resonator absorber. Conceptually, we have theoretically and experimentally demonstrated an ultralow-frequency absorber, which achieves average absorptance of 82% within the frequency ranges of 50–150 Hz. The total structural thickness is only 176 mm, which corresponds to 1/39 of the wavelength at 50 Hz. Our strategy leverages the synergistic effect between the IB and over-damped meta-atoms, which may enable efficient broadband absorption at extremely low frequencies.