Enhancing sound absorption for an acoustic metastructure with extended tubes at ultra-low frequency

A micro-perforated plate (MPP) as a typical acoustic absorber has ideal sound absorption performance at mid-frequency. A novel metastructure with a MPP and multiple sub-cavities (MPP-MSC) is proposed to broaden the sound absorption frequency range based on Helmholtz resonance, and its sound absorption performance is studied theoretically, numerically, and experimentally. Then, the genetic algorithm is adopted to extend the effective sound absorption band by 120 Hz and reduce the structure thickness to less than 61 mm, which is about 40% thinner than the existing structures. Additionally, a perforated plate with extended tubes (PPET) and porous sound absorptive material (PSAM) are introduced into the metastructure to enhance sound absorption performance in the ultra-low and mid-frequency range. Therefore, the composite sound absorber (PP-MPP-MSC) composed of PPET, PSAM, and MPP with superior sound absorption performance is proposed. Specifically, the effective acoustic absorption frequency of the structure can be broadened to 200–1800 Hz with a maximum thickness of 61 mm. It is worth noting that the design and fabrication of PP-MPP-MSC are relatively accessible, and effective sound absorption performance can be achieved in a certain frequency range, which presents the guiding role for current noise reduction applications.