We theoretically and experimentally demonstrate an approach to design a kind of broadband muffler with functionalities of sound insulation and ventilation at low frequencies. By using a bypass space coiling tunnel connected to a central open hole, a large bandgap at low frequencies is discovered in the periodic system. Due to the tunability of the tunnel, the labyrinthine metasurface possesses a strong ability to control the performance and the bandwidth of sound insulation. Through a cascaded array of the metasurfaces, the muffler can exhibit the properties of large transmission loss (TL) with broad bandwidth. A cylindrical muffler of height 15.6 cm and diameter 10 cm composed of a simple array of metasurfaces with exactly the same parameters is observed to produce broadband transmission loss (TL) with a peak of 75 dB at 300 Hz. Through optimization in different metasurfaces, TL in the whole spectrum from 198 to 412 Hz can be enhanced to be over 15 dB. This theory is verified by both band structure analysis and acoustic experiment. Our findings open an approach for low-frequency and wideband noise control in general exhaust systems. Published under an exclusive license by AIP Publishing.
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