Deep sub-wavelength broadband metasurface with micro-perforated panels and extended-neck Helmholtz resonators

This study develops, fabricates, and characterizes a deep sub-wavelength broadband acoustic metasurface for noise absorption. The metasurface is composed of micro-perforated panels coupled with extended-neck Helmholtz resonators, forming a unit absorber referred to as Helmholtz resonator absorber (MHA). Experimental results demonstrate that a 48 mm deep MHA achieves near-perfect sound absorption (>97%) at 150 Hz. This performance is realized with a sub-wavelength thickness of only 1/48 of the operating wavelength and a volume-normalized wavelength ratio of 1/54. Additionally, the MHA exhibits a half-absorption bandwidth of 48 Hz. To broaden the sound absorption bandwidth while keeping the total area constant and to further reduce the total thickness, a configuration integrating three Compact MHA (CMHA) units is proposed. Experimental results demonstrate that the CMHA achieves an average sound absorption coefficient (SAC) exceeding 0.74 in the 300–500 Hz range with an overall thickness of only 27.5 mm and an SAC > 0.88 in the 543–945 Hz range with a reduced thickness of 26.2 mm. Experimental results compare well with theoretical and numerical predictions, highlighting the potential of the proposed design for practical noise control applications.