Ultra-broadband sound-absorbing metastructure with Helmholtz resonator and porous material modulation crown

The development of sound-absorbing metamaterials/metastructures provides a new way to solve the problem of low-frequency and broadband sound absorption. However, designing ultra-broadband sound-absorbing metamaterials/metastructures with simple constructions is still a challenge. To address this problem, this paper proposes a type of hybrid metastructure (HMS) consisting of slit-embedded Helmholtz resonators (HRs) covered with porous material modulation crowns (PMMCs) for ultra-broadband efficient sound absorption, with virtually no upper limit on the absorption frequency. Based on double porosity theory, an analytical prediction method for the absorption performance of HMS is developed and validated by comparison with finite element simulations and experiments. Analysis of the absorption mechanism reveals that the PMMCs facilitate impedance matching and energy dissipation within the HMS. This not only enhances low-frequency absorption but also grants excellent tunability and absorption performance at mid-to-high frequencies. It is demonstrated that a typical HMS composed of only four subunits can achieve efficient ultra-broadband absorption (alpha >= 0.8) ranging from 100 Hz to more than 10000 Hz with a thickness of 200 mm. This work brings a new pathway for achieving lowfrequency and ultra-broadband sound absorption using metastuctures with simple and lightweight constructions.