Broadband and reconfigurable sound ventilation barriers at low-frequency with phase-gradient metamaterials

Abstract Acoustic metamaterials that enable sound insulation while maintaining excellent ventilation performance are of great interest. However, their bandwidth is typically very narrow, especially for low-frequency sounds. In this study, we propose a novel ventilated sound-proof acoustic metamaterial consisting of a phase-gradient array with space-coiling structures arranged within a square air passage. Due to the subwavelength response of the space-coiling structures (0.35 λ in the direction of ventilation), effective sound insulation is achieved in the low-frequency range ( < 500 Hz). The underlying physics can be attributed to anomalous reflection caused by the phase-gradient design, rather than the conventional mechanism of local resonance dissipation. Although the phase-gradient condition is only strictly satisfied at a specific frequency, the frequency range of sound transmission loss (STL) larger than 10 dB still extends to 452–553 Hz. The STL is even higher for obliquely incident sound waves and its performance can be further improved with the hybrid or folding structures. These findings open new avenues for applications, such as noise reduction in air vents of green buildings and machine cooling systems.