Low-frequency and broadband waterborne muffler based on low acoustic impedance arc-shaped metamaterials

Low-frequency noise control in ship seawater pipeline systems remains challenging, as mufflers are required to deliver good broadband performance within compact spaces. Inspired by bladder mufflers that exploit the strong impedance mismatch between low-impedance air and water for efficient sound reflection, this work introduces low-impedance arc-shaped metamaterials (ASMs) into conventional expansion-chamber mufflers and proposes a compact waterborne muffler. Based on different integration methods between the ASM and the expansion chamber, three muffler configurations are developed: Model B (circular chamber with annular ASM), Model C (circular chamber with revolved ASM), and Model D (square chamber with planar ASM). The equivalent acoustic parameters of the ASM are calculated and analyzed using homogenization theory. Results demonstrate that increasing the central angle of the arc-shaped beam or adjusting the deflection angle of the metamaterial significantly reduces its equivalent acoustic impedance, thereby enhancing low-frequency noise-reduction performance. Numerical simulations show that, compared with conventional expansion-chamber mufflers, the introduction of low-impedance ASM significantly improves the transmission loss (TL) in the low-frequency range and broadens the effective noise-reduction bandwidth. As a proof of concept, a muffler specimen based on Model B is fabricated and tested in a water-filled circulating pipeline system. The measured TL agrees well with simulations, yielding an average TL of 15.7 dB and an average insertion loss (IL) of 10.9 dB in the 200–2000 Hz band. This work provides a new approach for low-frequency noise control in fluid-filled pipeline systems and holds potential for applications in marine engineering and ocean equipment.