A phase-gradient acoustic metasurface for broadband duct noise attenuation in the presence of flow

A phase-gradient acoustic metasurface is designed based on wavefront manipulation for achieving broadband noise reduction in a flow duct. It is constructed by eight periodically arranged varying-depth units filled with porous materials, forming a linear phase shift of reflected wave from 0 to 2π at 3000Hz. The linear reflected phase-shifting can be roughly maintained in a wide frequency range. By employing the broadband phase-shift feature of the units, the metasurface exhibits an enhanced sound absorption performance in terms of the absorption coefficient and bandwidth when comparing to the individual units, which originates from forcing the higher-order modes of reflection waves to be evanescent. Then, its transmission attenuation characteristics in the presence of background flow are assessed by installing it on the side wall of a flow tube. The effects of flow speed (Mach number is up to 0.3) and sound source position (at the upstream and downstream sides of the metasurface) are experimentally and numerically investigated. Results show that the attenuation performance of the metasurface decreases with flow speed for the upstream sound source while increasing for the downstream sound source. For both upstream and downstream sound sources, a high transmission loss (≥20dB) is achieved by the metasurface in the frequency range of 1500–3000Hz at different flow speeds. Hence, the proposed metasurface is promising for achieving broadband noise attenuation in the presence of flow.