Modelling of parallel barrier for noise control

Abstract

Noise barriers are commonly used to protect the residents from the disturbance of traffic noise. Parallel barriers would also be used in highly populated area but its performance is deteriorated due to the resonances created by the multiple reflections between two parallel walls. To solve this problem, a Helmholtz resonator which acts a sound radiator is mounted on the wall surface to radiate sound and influence the structure of sound reflection waves inside the parallel barriers. Ultimately, the degradation effect due to resonance would be reduced. In order to understand the mechanism and design optimal Helmholtz resonator for controlling noise at wide frequency band, an analytical model for acoustical coupling of the baffled open cavity and the resonator array has been established. The resonators are regarded as the secondary sound sources which interact with multiple acoustic modes inside the cavity. Theoretical study indicates that sound peaks at outside receiver is dominated by one of the cavity modes and contributed from other modes. And the noise reduction inside and outside the cavity can be found at the target frequency when there is an appropriate design of resonators. The findings from the baffled open cavity in three dimensions is then applied to the noise control of the parallel barriers in two dimensions. There is no analytical expression to describe the acoustics field for open cavity without baffle and a hybrid method alternatively has been established by combining the analytical cavity modes and the numerical radiation modes. In order to suppress the multiple peaks of sound pressure levels at some frequencies, several resonators at different natural frequencies are combined together to obtain a broadband sound abatement. The validated hybrid model indicates that the noise reduction at the receiver can be found at the target frequency. Through optimal design of the locations of the resonators, the deterioration can be suppressed and a broadband noise reduction can be obtained. In order to have higher sound suppression especially at low frequency regime, the plate cavity device is installed on the inner walls of parallel barriers. The flexible panel is used to radiate sound to undergo sound cancellation at the region of the barrier top edges to suppress the sound diffractions. A theoretical model which account for the acoustical coupling between the plate vibration and sound radiation of the parallel barriers has been developed. The noise abatement of the parallel barriers after integration with the plate cavity is investigated systematically and optimized on the base of the validated theoretical model. It is found that the several sound peaks can be suppressed when the clamped-clamped plate is properly designed with light mass and high stiffness. To validate the accuracy of the proposed theoretical model for vibroacoustic coupling of such a complicated system, numerical tool of the fast multipole boundary element method (FMBEM) would also be established. This model can also be adopted to examine the effectiveness of the proposed silencing device in practical use and give the flexibility into the design of complicated structure of the parallel barriers in a three dimensional configuration. The BEM is the ideal solver for sound scattering in infinite space due to the fact that the Sommerfeld radiation condition can be satisfied inherently. The conventional BEM is insufficient for dealing with the sound scattering problem with large scale degree of freedoms and hence the fast multipole algorithm is chosen to accelerate the matrix vectors formulation and computation. Finally, the finite element method (FEM) is coupled to the FMBEM to deal with acoustic-structural interaction. Finally, a series of small scale experiments for open cavity and parallel barriers in anechoic chamber were conducted to valid the analytical and numerical models.