Narrow Results

The underwater acoustic siphon effect is proposed in this work, which aims to reveal the basic physical mechanism of high-efficiency sound absorption in meta-structures composed of multiple detuned units. Furthermore, the influence of the area ratio on the underwater acoustic siphon effect is then investigated by finite element simulation (FES) and theoretical calculation. On this basis, a meta-structure with the maximum absorption coefficient of almost 100% and average absorption coefficient of 80% at 600–1400 Hz is achieved. The underwater acoustic siphon effect could provide a better understanding of high-efficiency sound absorption and offer a new perspective in controlling underwater noises.

An ultra-thin waterborne acoustic metamaterial (AM), which is made of steel and composed of multi-Helmholtz resonators (MHRs), is proposed to achieve perfect sound absorption at low frequencies, which are generated around the resonance mode. The average surface acoustic impedance of the metamaterial is almost perfectly matched with water impedance under the action of resonance among the HRs, thus the perfect sound absorption is achieved. The case of two resonators is taken as an example to verify the design idea. By adjusting HRs’ sizes in simulation, the sound absorption coefficient reaches 99.6% at low frequency of 2740 Hz with ultra-thin thickness less than $\lambda /14$. The abnormal physical properties of AMs are often accompanied by abnormal effective material parameters, which turn to be negative near the perfect sound absorption through inversion calculation. The HRs proposed are simple to fabricate, mechanically stable, and convenient to couple with other resonators to achieve low-frequency broadband sound absorption.

In the present study, white, pink, construction and environmental noises were evaluated in a student hostel at National University of Singapore (NUS) with a newly designed sonic crystal (SC) window using sound quality head and torso simulator. The sound head was placed at the bed and the chair in the student hostel. All recorded data were analyzed into full (100–7000Hz) and narrow (700–1400Hz) frequency ranges. For lying position and for white noise, the SC window was able to attenuate extra 6.7 and 9.5dBA of noise compared to the existing glass louver window for full and narrow frequency ranges, respectively. For full frequency range, the overall amounts of pink, construction and environmental noises attenuated by the SC window were about 4.8, 5.0 and 3.4dBA, respectively. For narrow frequency range, the overall amounts of these three type of noises attenuated by the SC window were about 2–3.9dBA higher than that of full frequency range. The overall amounts of attenuated white noise at sitting position were about 2.6 and 6.1dBA, respectively, for full and narrow frequency ranges. For full frequency range, the attenuated pink and construction noises at sitting position were about 3.8 and 4.8dBA, respectively, which were slightly lower than that at lying position. However, for narrow frequency range, the attenuated noises obtained at lying position were 2.7 and 2.5dBA higher than that at sitting position for pink and construction noises, respectively.