Near-perfect sound absorptions in low-frequencies by varying compositions of porous labyrinthine acoustic metamaterial

Abstract

Low-frequency sound absorption is the key challenge in acoustics, and even the remarkable labyrinthine acoustic metamaterials with sound-hard boundaries are unable to attain an increase in the range of sound absorption under 500 Hz. This paper presents a new Porous Labyrinthine Acoustic Metamaterial (PLAM) that contains a folded slit labyrinthine structure in a micro-porous matrix, such that the matrix also participates in sound propagation. Theory, simulations, and experiments show that PLAM gives near-perfect sound absorptions at low frequencies (200–500 Hz) for different compositions. These near-perfect sound absorptions are due to particles vibrating in resonance in folded slit entry and dissipating acoustic energy into micro-porous matrix, which leads to zero acoustic pressure at the transmission end. For this purpose, labyrinthine structure and micro-porous matrix must acoustically couple, for which the characteristic frequency should be less than the resonance peak of the labyrinthine. Under these conditions, changing the Johnson-Champoux-Allard parameters of the micro-porous matrix changes the magnitude and frequency of the absorption peak. Increasing the number of folds of the labyrinthine structure shifts the sound absorption peak to lower frequencies but narrows the peak. However, parallel combination of PLAMs with different fold numbers, together, broadens the absorption peak at the low frequencies. The PLAM has been validated to attenuate noise level of an HVAC appliance by 6 dB without affecting the airflow performance. Hence, the presented work paves way for designing labyrinthine metamaterials with enhanced low-frequency sound absorptions and broader sound absorption range for aerodynamic noise control applications.