Labyrinthine type acoustic metamaterial with micro-slit wave entry of varying cross section for low frequency and broadband sound absorption

Absorption of low-frequency sound below 1000 Hz in air is essential in everyday life. Acoustic metamaterials, designed by coiling-up the space inside, address the challenges of increased size and poor low-frequency performance typically associated with conventional acoustic materials. One such metamaterial, featuring a micro-slit wave entry coupled with a labyrinthine air channel, has been shown to exhibit broadband low-frequency sound absorption due to surface impedance matching with the background medium, Fabry–Pérot-like resonances within the labyrinthine cavity, and thermo-viscous losses in the micro-slits. However, its effectiveness decreases at low frequencies due to an insufficient bandwidth and degradation in absorption magnitude. This article proposes a modification to the micro-slit, replacing its uniform cross section with a variable cross section shaped like a sine curve. When coupled with the labyrinthine air channel, this design enables perfect absorption at low frequencies. Theoretical and numerical analyses demonstrate that the proposed acoustic metamaterial can effectively absorb low-frequency sound (less than 350 Hz) over an extended frequency range—an achievement difficult to attain with conventional labyrinthine metamaterials. Furthermore, a periodic array of eight-unit cells of the proposed design exhibits nearly twice the absorption magnitude and a 60% increase in the absorption bandwidth (up to 55 Hz) compared to conventional designs, all while maintaining the resonance frequency at 300 Hz. By suitably tuning the variable cross section of the micro-slit—both its width and length—perfect sound absorption can be achieved across a sufficient frequency range at any low frequency below 1000 Hz. This work presents an effective approach to designing low-frequency broadband and subwavelength sound absorbers.