Acoustic metamaterial for highly efficient low-frequency impedance modulation by extensible design

We propose a novel class of acoustic metamaterial for low-frequency broadband sound absorption with high efficiency. The design principle is physically decoupled into low-frequency mitigation and working frequency widening. The embedded tubes with phase shift are employed to alleviate low-frequency noise, achieving a quasi-perfect absorption ( α >0.9) at 240 Hz with the peak value of 0.998. The spider-web-like configuration brings the high-efficiency acoustic impedance modulation via manipulating the coherent coupling effect, and allows for constantly broadening the effective working frequency in lower frequency domain by taming the resonant characteristics and the scale ratio of original and extended units. The theoretical and numerical methods are greatly validated by measured data. We further demonstrate the broadband potentials of the metamaterial, with a continuous quasi-perfect absorption from 301 Hz to 534 Hz (i.e., 1/22 of the operating wavelength) achieved from analytical results. The underlying physics is explained via coherent coupling, acoustic impedance matching, and system damping in detail. All in all, our work provides a viable and effective scheme in overcoming extremely low-frequency broadband noise for lightweight structures.