Low-frequency sound-absorbing metasurface constructed by a membrane-covered and coiled Helmholtz resonator

Achieving broadband absorption of sound waves below 500 Hz with materials of sub-wavelength thickness is significant but still a great challenge in academia and industries. Here, we present and theoretically analyze an airtight sound-absorbing metasurface constructed by a membrane-covered and coiled Helmholtz resonator. It is discovered that the metasurface possesses a near-perfect absorption with a working wavelength approximately 33.6 times greater than the total thickness, which stems from synthetic modulation on acoustic reactance brought by the membrane, air gap formed behind the membrane, and a coiled channel. Furthermore, on-demand broadband absorption below 500 Hz is achieved by parallel assemblies consisting of four subunits. An excellent agreement between measurements and predictions confirms the validity of the proposed structures. The airtight construction also broadens its application scenarios compared to the common perforated absorbers with open pores directly exposed to external environments. Our design provides a new structure paradigm for low-frequency sound absorption.