Broadband low-frequency sound absorption of multi-layer stacking metamaterial with non-degenerate eigenmodes

We propose and validate numerically and experimentally an ultra-broadband low-frequency sound absorption unit cell composed of three layers of sub-units, which consist of central rectangular cavities surrounded by coiled channels outside. The broadband sound absorption effect arises from the coupling of the non-degenerate eigenmodes of the designed unit cell, whose eigenfrequencies are distributed over a wide region in the low-frequency domain. The noise energy is mainly dissipated in the coiled channels due to viscous losses. The fractional bandwidth (the ratio of the bandwidth to the center frequency) of the proposed unit cell is about 37.9%. The working band can be modulated by adjusting the structure parameters of the unit cell. In addition, we further improve the fractional bandwidth by combining two unit cells with different parameters, which reaches about 48.4% (nearly 50.0%), showing an ultra-broadband characteristic. Our proposed design takes the advantages of sub-wavelength, reconfigurable structure and ultra-broad working band in the low-frequency range, paving a new way to design broadband low-frequency sound absorption devices, and may have great potential applications in the fields of low-frequency noise control and architectural acoustics.