Tunable parallel barriers using Helmholtz resonator

Abstract

Parallel barriers are widely adopted to control environmental noise, but their performance may be inferior to that of a single barrier owing to the formation of multiple reflection waves between the parallel barriers. To improve the performance of parallel barriers, single or multiple Helmholtz resonators (HRs) are proposed to be mounted on the barrier surface. An acoustic interaction occurs between the HR and open cavity formed by the rigid ground and a pair of barriers, whereby the acoustic modal response within the open cavity is significantly suppressed and the diffraction wave at the barrier top edge is reduced. A semi-analytical model for dealing with acoustic coupling between the open cavity and HRs in a two-dimensional configuration is established in order to understand the sound suppression mechanism within the shadow zone. This model is also helpful for generating a noise control strategy that involves varying the dominant modal response at the target frequency. With the optimal position of a single HR, the insertion loss of about 10 dB around the target frequency can be controlled, while less influence is exerted on the off-target frequency. Comparisons are conducted between the data predicted using the present model and the numerical results obtained using the boundary element method, and the agreement between them is observed. Furthermore, experimental results demonstrate that the use of HRs for reducing noise in the shadow zone is feasible.