Innovative compact low-frequency absorber design: Integrating power-law-varying aperture into Helmholtz resonator

This study introduces an innovative design that integrates an acoustic black hole (ABH) structure into the embedded aperture (EA) of a Helmholtz resonator (HR), where the EA radius follows a power-law function. Analytical models and numerical simulations, based on the transfer matrix method combined with the equivalent fluid model and finite element method, were developed to investigate the absorption performance and underlying mechanisms, as well as the impact of EA geometric parameters on absorption. Experimental tests validated the models, demonstrating good consistency. Results show that the ABH-shaped EA significantly improves absorption performance compared to cylindrical and conical EAs. The absorption peak increases by 23%–60%, and the peak frequency decreases by 18%–29%, achieving quasi-perfect absorption (up to 0.9 experimentally) with a thickness of only λ/28 at target frequencies. The incorporation of the ABH significantly enhances energy dissipation within the EA, optimizes surface acoustic impedance, increases the absorption peak, and shifts the peak frequency to lower values. By adjusting EA parameters such as the power-law exponent, radius, and length, the structure can achieve perfect absorption (reaching 0.99 in simulations) in the low-frequency band without altering the external shape. Additionally, replacing cylindrical EAs with ABH-shaped EAs in an acoustic metasurface, which is composed of parallel HR units, will reduce the peak frequency by 8% and increase the quasi-perfect absorption bandwidth by 12.5%. This design offers a promising approach for developing compact, wideband, and efficient low-frequency sound absorption structures.