Adjustable sound absorbing metastructures for low-frequency variable discrete sources

Low-frequency discrete variable noise sources, such as those in engines and fans, are frequently encountered in daily life. Due to the variability in noise frequency and the inherent characteristics of longer wavelengths, traditional acoustic structures face challenges in effectively absorbing such variable noise below 600 Hz. To address this, we propose using multiple-frequency adjustable sound absorption elements in parallel for variable low-frequency noise absorption. Each element's absorption frequency can be adjusted by modifying its bottom cavity volume. A theoretical model based on Helmholtz resonance is developed and validated through simulations and experiments, achieving high-efficiency sound absorption (alpha > 0.95) in the 260-520 Hz range. This model quantifies the dependence of absorption efficiency on design parameters and explores underlying mechanisms. For turbomachinery, a metastructure combines multiple elements with adjustable cavity volumes to broaden absorption bandwidth. Compared to non-adjustable materials, our metastructure achieves superior sound absorption below 600 Hz, broadening effective bandwidth by over 30 %. Limitations in extreme environments are discussed, suggesting potential improvements. This work can provide inspiration for the design of novel sound-absorbing structures with intelligent control of low-frequency variable noise and matching control systems.