Actively tunable sound absorbing porous Helmholtz resonators

Abstract Tunable sound absorbers are used to reduce spectrum-varying noise and build controllable anechoic boundaries for virtual reality applications. Conventional mechanically tunable sound absorbers are manually operated, and typically fail to maintain a high absorption peak at new target frequencies by changing only a single structural parameter. To address this issue, an actively tunable sound absorber is developed, consisting of multiple rotatable perforated panels and a fixed perforated panel backed with a perforated porous material panel as a tunable absorber, microphones as sensors, a stepper motor as an actuator, and a computer as a control system. The absorption peak frequency is adjusted by varying the overlapping area of the orifices on the rotatable and fixed perforated panels, according to the real-time noise spectrum. The air in the overlapping area and that in the perforation holes of the perforated porous material act as the air mass and air spring, respectively, of a Helmholtz resonator. A theoretical model is first developed for the initial state based on the double porosity theory. Finite element simulations reveal that sound waves strongly diffuse into resistive porous materials at the Helmholtz resonance frequency, leading to large energy dissipation. This compensates the insufficient energy dissipation around the too large overlapping area, ensuring near-perfect absorption across a wide range of overlapping areas. The experimental results demonstrate that near-perfect absorption performances are achieved across the rotation angle from 0° to 37.8°, corresponding to the decline of the overlapping area and a peak frequency range from 670 Hz to 426 Hz with peak coefficients exceeding 0.989. Based on the measured absorption spectra at different rotation angles, an automatic-regulation experiment is conducted for varying single-frequency noise. This novel integration of tunable absorbers, sensors, actuators, and a control system offers a practical pathway for transitioning conventional manually tunable absorbers into actively tunable absorbers.