An ultra-low-frequency sound absorber and its application in noise reduction in ducts

We demonstrate, both numerically and experimentally, an ultra-low-frequency sound absorber and its application in silenced ducts. The absorber comprises an array of resonant dual-spiral channel units (with a thickness of lambda/47) backed by a wall, achieving near-perfect sound absorption with a fractional bandwidth of 18.3% around 73 Hz-a hallmark of ultra-low-frequency sound absorption. This characteristic arises from its effective near-zero modulus, with the absorbed energy dissipated through viscous losses in the unit channels. Furthermore, we explore the application of this ultra-low-frequency absorber in silenced ducts. By designing a composite absorber of five units with different parameters, we experimentally demonstrate efficient sound absorption in a duct with an enhanced fractional bandwidth of 60.6%. The average absorptance approaches 0.87. This designed absorber offers the advantages of deep-subwavelength thickness, ultra-low-frequency sound absorption, and broad bandwidth, opening up new possibilities for metamaterial-based absorbers in practical applications.