Acoustic characterizations of Helmholtz resonators with extended necks and their checkerboard combination for sound absorption

Abstract

This study presents a thin absorber for low-frequency noise mitigation based on a Helmholtz resonator with an extended neck (HREN). An analytical model is established based on the transfer matrix method and the equivalent medium model to predict the acoustic characteristics of the HREN-based absorber, and is validated by experiments and simulations. The acoustic properties of the HREN are characterized comprehensively, including the effects of the radius and length of the extended neck. It is demonstrated that a feature of the HREN is its thin thickness (around 1/30th wavelength of the operating frequency), and its resonance frequency can be flexibly tuned by adjusting the geometry of the extended neck. The HREN can achieve effective low-frequency absorption in a constricted space, but is only effective within a narrow bandwidth. To overcome the limitation, a checkerboard absorber consisting of alternately distributed inhomogeneous HRENs is investigated. Results show that the HRENs in a checkerboard absorber operate almost independently, resulting in a dual-band sound absorber, when the adjacent HREN units are largely dissimilar. In contrast, a bandwidth extension of effective absorption can be obtained if the adjacent HRENs in the checkerboard absorber are strongly coupled. The absorber with a broadened absorption bandwidth exhibits a quasi-perfect absorption (the absorption coefficient above 0.85) within the bandwidth between the two absorption peaks induced by two corresponding HRENs.

Progress of low-frequency sound absorption research utilizing intelligent materials and acoustic metamaterials

“…The absorption coefficient of this absorber in the band of 819.2 Hz to 919.2 Hz reached up to 0.85, achieving quasi-perfect sound absorption. 44 With the above principle, an absorber with deep subwavelength thickness was designed by assembling multiple non-uniform layers of extended-neck Helmholtz resonators with overlapping absorption peaks induced by different units, resulting in a continuous absorption band in the low frequency region. 45 In addition to the regulation and control of structural parameters, the absorption bandwidth of Helmholtz resonators can also be broadened in the low frequency band by the design of hybrid structures with the incorporation of other acoustic structures.…”

“…The sound absorption characteristics of MPP absorbers are closely related to the perforation rate, shape, cross-sectional structure, dimension, and distribution of the pores. 44 (b) Schematic of a checkerboard absorber composed of alternating resonators A and B with varying-length extended necks. 44 (c) Schematic of the complete compliant-material resonator and a cross-section.…”

“…44 (b) Schematic of a checkerboard absorber composed of alternating resonators A and B with varying-length extended necks. 44 (c) Schematic of the complete compliant-material resonator and a cross-section. 46 Optimizing these parameters could regulate the operating frequency, improve the absorption performance, and broaden the effective frequency band.…”

“…Fig.1Helmholtz resonance structures. (a) The cross-sectional view of a Helmholtz resonator unit with an extended neck 44. (b) Schematic of a checkerboard absorber composed of alternating resonators A and B with varying-length extended necks 44.…”

“…(a) The cross-sectional view of a Helmholtz resonator unit with an extended neck 44. (b) Schematic of a checkerboard absorber composed of alternating resonators A and B with varying-length extended necks 44. (c) Schematic of the complete compliant-material resonator and a cross-section 46.…”