Incorporating extended neck Helmholtz resonators into an advanced multi-degree of freedom acoustic metamaterial for low-frequency, broadband acoustic absorption

Abstract

This study investigates the novel design and evaluation of a low-frequency, broadband, noise-absorbing acoustic metamaterial. The technology combines the sound-absorbing characteristics of micro-perforated panels with extended neck Helmholtz resonators. Individual sub-chambers can have unique properties such as depth, porosity, hole diameter, extended neck length, and neck internal diameter. An optimisation algorithm is implemented to ensure that broadband absorption is maximised in a low-frequency range of interest. The Helmholtz resonators couple with the response of the micro-perforated panels so that the combined response is both more broadband and of lower frequency than could be attained by the elements individually. Low frequencies below 500 Hz are of interest in the study, and the area exposed to the sound field is limited to 0.04 m2, so the technology is considered to be a locally reacting absorber. Experimental results indicate that perfect sound absorption of over 99.9% at 329.6 Hz with a 51 mm thickness (sub-wavelength ratio = 1/20.4) can be achieved with one configuration, which emphasises maximum absorption. A second configuration, which emphasises broadband absorption, achieves an average sound absorption coefficient of 0.8 in the 254 Hz to 500 Hz range. Experimental results compare well with analytical and numerical results and outperform some results found in the literature.