Control of absorptive and reflective mechanism to optimize noise barrier design

Abstract

Noise generated from transportation systems can be effectively reduced by the erection of absorptive barriers. However, environmental engineers have long been aware of the conflict between the acoustic performance and structural durability of the absorptive material held inside the barriers. To optimize the design, the abatement of the diffraction noise at top of the barrier is not the only consideration, but the reflection noise is also an important factor. In this dissertation, it is attempted to combine the advantages of absorptive and diffusive mechanism on the barrier to reduce the diffraction and reflection noise. From the experimental result, it is found that the reflecting property of high frequency sound wave is similar with the optical wave. However, the reflecting property of low frequency sound wave is relatively complicated. For the prediction of the reflection effect, the image source with virtual barrier model is developed. This program can locate the critical reflection zone with the different tilted angles of barrier. With the aid of the data, a partial tilted barrier with special panel absorber is designed. The special panel absorber composes of a vibrating metal membrane which is fixed tightly on a structural frame and the narrow air backing partitions inside. Since the span of the new design is shortened and the boundary is fixed, the stiffness and damping are relatively higher than the traditional one. Thus design of panel absorber utilizing structural damping instead of air damping improves the structural durability and acoustic absorption performance. To abate the diffraction noise from the top of barriers, the extended V-shape barrier is developed. The upper part of barrier towards is extended the side near of source to increase the height of shadow zone behind the barrier. The concept of V-shape device is providing two separates obstacle structure along the line of sight between source and receiver, the sound is diffracted over the top edges of both barriers and absorbed in the gap between the barriers. By this device, both the scale model and the prediction results show that 5dB additional insertion loss can be attenuated. After these researches, the composite barrier design is developed. The acoustic performance of the composite barrier is comparable to partial enclosures for environmental noise control and can be structurally durable.