Parallel-coupled hierarchical and reconfigurable structure for broadband sound absorption

Conventional series-coupled or parallel-coupled sound absorbers can only work effectively in a fixed frequency range, of which the absorption spectra can not be allowed to divide. Here, a parallel-coupled hierarchical structure with two relatively independent frequency bands of sound absorption is proposed. The hierarchical structure is composed of two type subunits of embedding neck Helmholtz resonator (ENHR) and segmented neck Helmholtz resonator (SNHR). Sound energy in the high-frequency region is dissipated in the first layer by the multiple local resonances coupling of ENHRs. The second layer effectively captures and absorbs low-frequency energy through the coupling of multiple SNHRs with varying parameters. The comprehensive discussion on the effects of acoustic impedance and structural parameters highlights the superior capability of this structure in impedance modulation. To demonstrate its broadband absorption performance, a hierarchical structure consisting of 20 subunits is employed. Finally, a above 90% broadband quasi-perfect absorption over a frequency range from 450 Hz to 1750 Hz is achieved under a thickness of 48 mm. Furthermore, a dual-band modular design concept for frequency-selective absorption is presented based on the tunability and independence of the two absorption bands. Therefore, the proposed hierarchical structure achieves reconfigurability by replacing either the first or second layer, thereby breaking away from traditional parallel design methods and providing a practical approach to controlling noise sources whose spectral characteristics change with operating conditions.