Expanding the strong absorption band by impedance matched mosquito-coil-like acoustic metamaterials.

A mosquito-coil-like acoustic artificial structure consisting of a spiral channel and a perforated plate with excellent impedance matching is proposed, which can realize strong sound absorption within a certain frequency range. Due to the difficulty in matching the impedance of the single-hole structure with that of the sound propagation medium, the sound absorption should be poor. To overcome this shortcoming caused by the mismatched impedance, some multi-hole microstructures are designed. Moreover, since single-chamber labyrinth can only achieve single-frequency perfect sound absorption, a labyrinthine channel is divided into several chambers with each length distributing by an arithmetic progression gradient. The sound absorption bandwidth can be extended by synergetic coupling resonance among multiple chambers. By selecting different structural parameters including the number of holes, the width of the labyrinthine channel, and the depth of labyrinthine channel, sound absorption of these mosquito-coil-like structures is investigated. The results suggest that the multi-hole structures are helpful in improving the impedance matching, while the synergetic coupling resonance among multiple chambers ensures that the sound absorption coefficient of the structure can be maintained at a high level within a certain frequency range. In addition, some mosquito-coil-like sound absorption structures are fabricated by 3D printing, then the sound absorptions under vertical sound incident conditions are measured, and the strong sound absorption ability in a wide band is experimentally demonstrated. Finally, a method is proposed for adjusting the sound absorptions by proportionally zooming in or out the structure, by which the sound absorptions of the acoustic structure can be effectively shifted to lower or higher frequencies.