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

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.