Abstract
A broadband sound absorption attained by a deep-subwavelength structure is of great interest to the noise control community especially for extremely low frequencies (20-100 Hz) in room acoustics. In this paper, we report on an analytical, numerical and experimental study of a low-frequency broadband (50-63 Hz, one third octave band), high absorption (average absorption coefficient ≈93%), near-omnidirectional (0° - 75°) acoustic metasurface absorber at a thickness of 15.4 cm (1/45 of the wavelength at 50 Hz). To further broaden the bandwidth (50-100 Hz, one octave band), another design is analytically studied to achieve an average absorption coefficient of 85% for a wide angle range (0°-75°) at a thickness of 20 cm (1/34 of wavelength at 50 Hz). The proposed design methodology may solve the long-standing issue for low frequency absorption in room acoustics.
Highlights
A sound absorber with a broadband and high absorption at a deep-subwavelength scale is of great interest in many occasions, such as room acoustics (Cox and D’Antonio, 2016), automobiles and aerospace engineering (Nark et al, 2018)
The first type of numerical model is distinct from the second type of numerical model, which will be discussed in The Coupling Through the Evanescent Wave and Acoustic-Structure Interaction
Starting from around 50°, only three absorption peaks appears instead of four peaks in the absorption spectrum, which is a signature of coupling between two unit cells (Li et al, 2016) and this phenomenon is resulted from a coalescence of eigenstates (Ding et al, 2016; Wang et al, 2019) as the incident angle changes
Summary
A sound absorber with a broadband and high absorption at a deep-subwavelength scale is of great interest in many occasions, such as room acoustics (Cox and D’Antonio, 2016), automobiles and aerospace engineering (Nark et al, 2018). A particular interest is to realize the so-called modal equalization (i.e., absorbing the normal mode frequencies of a room which usually fall below 100 Hz) (Fuchs et al, 2001; Błaszak, 2008; Rivet et al, 2012; Lau and Powell, 2018) for improving sound generation and speech interpretation This is hindered by the inability of conventional sound absorbing materials to effectively remove low frequency sound (Yang and Sheng, 2017). To achieve the deep-subwavelength scale, one strategy is to use a very thin decorated membrane (Mei et al, 2012; Yang et al, 2015) In such a design, a uniform and controlled tension of the membranes is needed, which leads to fabrication challenges and durability issues. Another strategy is to modify the geometry of the conventional Helmholtz resonator (HR) and the microperforated panel (MPP) (Maa, 1998) into space-coiling structures (Cai et al, 2016; Li and Assouar, 2016; Huang et al, 2018), embedded-neck structures (Simon, 2018; Huang et al, 2019) or multi-coiled structures
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