Ultra-thin and broadband low-frequency underwater acoustic meta-absorber

Acoustic metamaterials with deep-subwavelength thickness have aroused increasing interests for potential applications in low-frequency sound and vibration control. Most reported metamaterials, however, are for airborne sound, with fewer for low-frequency waterborne sound absorption because of water's much longer wavelength, weaker dissipation and closer impedance to solids. Current underwater sound absorption (SA) approaches merely work at broadband high frequencies (typically above 2 kHz) or narrowband low frequencies (by introducing discrete narrowband spring-mass local resonators (LRs)). Herein, an ultra-thin meta-absorber is proposed to achieve broadband low-frequency underwater SA via inserting thin and thickness-graded circular-elastic-plate scatterers (CPSs) into an elastomer matrix. Capitalizing on the thickness gradient among the CPSs and a backing plate behind the elastomer, the proposed design entails continuous broadband LRs, enriches the content of both local and coupled resonance modes inside the meta-absorber unit and enhances the coupling among them, thus enabling high and quasi-perfect SA at multiple frequencies and broad low-frequency range with a deep subwavelength thickness. Notably, quasi-perfect SA (>0.97) is realized at 415 Hz with an absorber whose thickness is 1.7% of the sound wavelength. An optimized design yields excellent sound absorption (>0.9, 0.952 on average) in the low frequency range from 500 to 2000 Hz. Such broadband low-frequency SA is confirmed by experiments. This research offers a novel and effective solution to achieve broadband low-frequency underwater SA, which may open up a new avenue to broadband low-frequency sound control using sub-wavelength structures.