Wideband low-frequency sound absorption by inhomogeneous multi-layer resonators with extended necks

This study develops a structure with thin thickness targeting for wideband low-frequency sound absorption, which consists of multi-layer Helmholtz resonators with extended necks (HRENs). An analytical prediction model is established based on the classic transfer matrix method to study the acoustic characteristics of the multi-layer HREN absorber. Results show that the added layer shifts the first absorption peak of the original single-layer HREN absorber to a lower frequency and induces additional absorption peak. Then, a particle swarm optimization (PSO) algorithm is coupled with the prediction model to design optimal sound absorbing structure for effective sound absorption in a prescribed wideband frequency range. For illustration, a single layer, a double-layer and a triple-layer optimal structures composed of 9 inhomogeneous units at each layer are designed and they yield about 90% absorption in the frequency ranges of [550,700] Hz, [400,650] Hz and [300,550] Hz. The mechanism of the continuous absorption band is due to the overlapping of the absorption peaks induced by different units. The proposed structure offers an effective acoustic absorption in the target wideband frequency range while its size is in a compact scale, and thus holds great potential for noise attenuation in various realistic applications.