Acoustically driven air vibration in cavity and its application to sound barriers

Abstract

Recently, due to rapid urban development and increasing population in modern cities, main traffic roads are closed to residential regions and lead to serious noise pollution. The annoyance from traffic noise brings health problems to citizens such as sleep disturbance, hypertension, and even ischemic heart diseases. Thus, road traffic noise becomes one of the critical problems in modern cities. Many acoustical researchers, environmental engineers and scientists pay more concerns on traffic noise problems and seek related solutions. One of the common solutions is to locate an obstacle between traffic road and residential region, which is so-called noise barrier. Based on the physical phenomenon of sound propagation, noise barrier can achieve high noise attenuation at shadow zone by blocking the direct propagation path from noise source to receiver. However, noise barrier has its limitation on low frequency noise due to the high diffractive efficiency of the latter. Low frequency noise attenuation level is thus poor. Improvement of the noise attenuation of noise barrier has then become a main research focus. Theoretical and experimental investigations have been conducted for half a century. It is found that barrier dimension and shape of barrier top edge would affect noise attenuation efficiency. Because of space limitation in densely populated cities, increasing the size of noise barrier is not a good solution to improve noise reduction performance of noise barrier. Different barrier top edge designs are then considered to achieve higher noise attenuation. From recent researches (Maekawa,1968) (Seznec,1980) (Watts,1996) (Ishizuka and Fujiwara,2004), numerical and experimental results also show that the general T-shape, Y-shape and cranked barrier can provide good noise attenuation the same as that of a higher and thicker barrier. If absorption material is added on the top edge of these barriers, noise reduction performance can be further improved. However, the performance of absorption materials always depends on atmospheric conditions and decrease dramatically in a short period after exposing to bad atmospheric environment. Diffusive barrier is then proposed to reduce noise by sound diffraction at barrier leading edge instead of absorption by absorption material. Different diffusive barrier designs are proposed in recent researches (such as Lam, 1994) to optimize the noise reduction performance. Moreover, studies on resonator (Ingard, 1953) (Tang, 1973) have been conducted for decades. Although the noise attenuation level of resonator is frequency dependent, the effective frequency range can be enlarged by using multiple resonators together (Doria, 1995) (Griffin,2001) (XU,2010). Therefore, a noise barrier associated with resonator is then being considered in this study. The major objectives of this study are to investigate the spatial behavior of sound behind barrier and noise attenuation performance of noise barrier with acoustic cavities on its top edge. In this research, measurements are carried out to indicate the relation of noise attenuation to the following parameters, which are dimension of cavity, arrangement of cavities, number of cavities used and location of cavity. Numerical computations are done in Chapter 4. The results show that the noise attenuation performance of a conventional vertical barrier is improved by adding a single acoustic cavity on its top edge especially at the resonance frequencies of acoustic cavity. In addition, the results also show that the magnitude of Insertion Loss depends on the location of acoustic cavity. When the distance between barrier leading edge and acoustic cavity is decreased, the magnitude of Insertion Loss is increased without influencing resonance frequency. Analysis of experimental results is shown in Chapter 5. The noise attenuation performance by different cavity arrangements is then investigated. Transfer function is used in these analyses to obtain the insertion loss. Conclusion can be drawn from overall experimental results that the separation from the cavity to the leading edge affects the magnitude of Insertion Loss significantly. Moreover, the resonance frequency of noise barrier is controlled by cavity depth especially at low frequency.