Low-frequency broadband absorbers based on coupling micro-perforated panel and space-curling chamber

Abstract

<p indent=0mm>Conventional acoustic absorbers, which are made of porous and fibrous materials, usually have a structural thickness comparable to the working wavelength, which inevitably hinders their applications in low frequency range. A micro-perforated panel with a backward cavity has high efficient absorption in low frequency region, yet still processes a bulky size comparable to the resonant wavelength. In the past two decades, metamaterials and metasurfaces, which possess the functionalities cannot be achieved by the natural materials, such as deep-subwavelength thickness, negative mass density or bulk modulus, negative refraction, etc., have got continuous attention and hence rapidly developed. Using the compact structures, such as membrane-type acoustic metamaterials, curling-up space structures, Helmholtz resonators with embedded apertures, to realize perfect absorption in low frequency has broadened the horizon of the acoustic absorption and provided a novel way for noise control. However, because of the dispersive nature of resonance, the aforementioned perfect absorbers cannot achieve broadband high absorption, which unavoidably confines their application. It is known that a micro-perforated panel features a broadband characteristic, while curling-up space structures have the capacity in absorbing the low frequency waves. Therefore, we come up with the idea of utilizing the micro-perforated panel and curling-up chamber to realize the high absorption in low and broad frequency region. In this work, we propose a hybrid absorber. By introducing the concept of curling chambers and coupling them to micro-perforated panels, the proposed acoustic absorber possesses remarkable properties such as deep-subwavelength thickness, broad absorption band and high absorption efficiency. Theoretical analyses, numerical simulations and experiments are carried out to reveal the underlying physical mechanism of the acoustic absorbers, whose results showed excellent agreements with each other. First, we theoretically analyze the absorption performance of each component consisting of a micro-perforated panel and a curling chamber. Then, we conduct the numerical validation and operate the experiment to verify the theoretical model. Through numerical optimizations, we finally design the two components achieving perfect absorption at lower <sc>(282 Hz)</sc> and higher <sc>(429 Hz)</sc> frequencies, respectively. Using the coupled mode theory, we present an absorber coupling the low- and high-frequency components. The hybrid absorber possesses the ability that the absorption coefficient is larger than 0.5 in the frequency range from 232 to <sc>533 Hz,</sc> and larger than 0.8 from 262 to <sc>469 Hz,</sc> while its structural thickness is only 10 cm (<italic>λ</italic>/12,<italic> λ</italic> is the working wavelength corresponding to the lower resonant frequency). To understand the underlying physical mechanism, we use the commercial software COMSOL MULTIPHYSICS to show the interaction (i.e., coupling) between the two components. As shown by the simulated results, at the resonant frequency of each component <sc>(282</sc> or 429 Hz), when planar wave impinges normally on the whole absorber surface, only the corresponding component has sound energy exchange with the outside. However, at the frequency between the two resonant frequencies, both of them switch sound energy with the outside, which demonstrates the coupling effect. In this way, the hybrid absorber can realize the broadband high absorption in low frequency range. Compared to traditional bulky structures, the proposed absorber possesses the ability such as easy to fabrication, deep subwavelength and broadband high absorption, and hence may be widely used in noise control engineering.