Broadband Acoustic Ventilation Barriers

Conventional sound barriers impede airflow at the same time. Recent advances in acoustic metasurfaces provide a solution for air-permeable barriers utilizing the Fano-like interference. While the mechanism of Fano-like interference implies that such a realized device serves a narrow working frequency range around every destructive-interference frequency. Considering the fact that noise usually covers a wide frequency range, designing a broadband acoustic barrier is still a challenge. Here, we theoretically design a planar-profile and subwavelength-thickness (approximately $\ensuremath{\lambda}/8$) acoustic ventilation barrier prohibitive for sound in a broad range. Our design is a metasurface consisting of a central hollow orifice and two surrounding helical pathways with varying pitch. Thanks to the hornlike helical pathways, the response strength from the monopolar and dipolar modes of the system almost keeps balance in the frequency range of interest, leading to an effective blocking of more than $90\mathrm{%}$ of incident energy in the range of 900--1418 Hz. Experiments are conducted to validate the proposed design, whose results are consistent with the analytical predictions and simulations. The underlying working mechanism ensures the metasurface is capable of handling broadband sound coming from various directions. Our design has potential in air-permeable yet sound-proofing applications, such as simultaneously natural ventilation and noise reduction in green buildings.