A low-frequency sound absorber based on micro-slit and coiled cavity

The design of a compact low-frequency acoustic absorber whose thickness is less than the operating frequency wavelength is a difficult challenge. However, recent developments in metamaterial engineering have enabled significant advances towards this goal. This paper presents an investigation of a low-frequency absorber (i.e., operating between 100 and 600 Hz) structured around a new concept of a thin cavity of coiled spaces coupled with a panel and micro-slit. The ability of this acoustic metamaterial absorber to control sound energy with increased relative bandwidth is examined analytically and numerically considering the viscous-thermal losses in both the micro-slit and coiled spaces. A prototype absorber was manufactured using advanced additive manufacturing technique and evaluated in an impedance tube apparatus. The experimental results showed the absorber capable of controlling 95% of the sound energy at 365 Hz with a relative bandwidth of 60.3%. These results compare well to those obtained from the analytical analysis and finite element method. Thus, the proposed acoustic absorber is shown to be capable of operating in the deep-subwavelength scale.