Tunable acoustic metamaterial for broadband low-frequency sound absorption by continuous acoustic impedance manipulation

In this paper, a continuously tunable acoustic metamaterial is proposed for broadband low-frequency sound absorption. The metamaterial can be considered as a reconfigurable Helmholtz resonator with an upper perforated panel that can be continuously rotated to adjust the neck length. A theoretical model is developed to study the tunable sound absorption performance of the metamaterial, which demonstrates that it exhibits efficient low-frequency sound absorption ( α > 0.9 ) in the frequency range (from 186 Hz to 319 Hz). Furthermore, perfect sound absorption ( α > 0.99 ) is achieved at 197 Hz with a deep subwavelength thickness ( λ / 56 ). To validate this model, numerical simulations and experimental measurements are performed, and the physical mechanism of tunable sound absorption is explored. Results show that the acoustic impedance manipulation is realized by rotating the perforated panel to tune the acoustic mass of the Helmholtz resonator, which is the key to the continuous tunability of the proposed metamaterial. Based on the continuous tunability of each metamaterial unit, a tunable broadband sound absorber is designed to achieve a variety of broadband sound absorption performance with a single fabricated structure. This work paves the way for the manual realization of flexible tunability of sound absorbers and helps to achieve tunable absorption of complex noise according to variable acoustic application scenarios.