Study On Noise Attenuation And It's Application By Tree Belts Along Highway

The fast development of highways promoteed tremendously the economics of the passing area. However, serious acoustic environmental pollution to the nearby habitants was brought in by the noise emitted from the vehicles also. The dominant methods to control the noise's transmission were setting up acoustic obstacles between the sensor points and the highway. As acoustic obstacle, the tree belt with sufficient width could reduce the noise effectively. However, the current mechanism of tree belt reduction noise was not yet entirely clear. On the one hand, in the noise evaluation, because of the existing tree belt sound attenuation expression was too simplified to reflect the actual characteristics of tree belt, the noise prediction results often had a greater error. On the other hand, because of the lack of quantitative basis, it was difficult to applied tree belt to the actual road noise reduction project. In this paper, based on fundamental acoustic principles, a systematic study of tree belts' noise-reducing effect, mechanism, quantification and design were executed. The main research contents of the present wok were as follows:1. Acoustic attenuation effects and its influencing factors of tree belt. Noise spectral analyzer, HS6288B, was used to monitor in situ the noise levels along a highway before and after various kinds of tree belts, in summer and in winter respectively. By analyzing the acoustic attenuation effects and influential factors, it is found in this thesis that the attenuation amount increases nonlinearly with the width, the crown density and the height of the tree belts. By comparing the attenuation effects of the tree belts in different seasons, it is found that evergreen tree belts of 30m wide could effectively reduce the traffic noise energy both in the winter and in the summer.2. Mechanism of attenuation sound energy of tree belt. In this thesis the spectral curves of acoustic energy attenuation by tree belts were analyzed and correlated to the internal microscopic structures of the plants. It is found that different tissue structures in the plants have varied acoustic attenuation frequency. The resonance sound-absorbing structures in the stem attenuate mainly the noise with frequency below 500Hz. While the multi-pore sound-absorbing structures in leaves attenuate mainly the noise from 500Hz to 8kHz. The sound-scattering effect of tree belts contribute to the attenuation at even higher frequency.3. Sound attenuation formula of tree belt. Based on the analysis to the acoustic absorbing and scattering mechanisms of tree belts, the formula to calculate the noise-reduction by tree belt was deduced in this thesis according to the transmission principles of acoustic wave through forest. In this formula, various kinds of influential factors, such as the tree belt's total width and height, the diameter of a tree crown, plant intervals, height of lowest branches, distance between the tree belt and the central line of the highway, and plant species and season, etc, had been considered comprehensively. Applying this formula into the traffic noise prediction models, it is found that the predicting accuracy is apparently superior to other formulas of the similar kind by comparing the real noise test results in summer and winter. Based on this, the author recommends using this formula as the official predicting formula to calculate the attenuation amounts of highway traffic noise transmitted through tree belts of various kinds.4. Design and application of noise reduction of tree belt, by researching the effects of many noise-reducing parameters like tree belts' width, height and length, the general principles of noise-reducing design and related applying condition were provided in this thesis. The noise-reducing design of tree belt was applied to practical engineering. According to the results, the noise-reducing effects of the tree belt designed for Ji-Hun highway had been well predicted, and achieving the purpose of precise control to the noise levels behind the tree belts. Thus providing a good guidance to the noise-reducing design of the practical projects.5. Coordination of tree belt and landscape ecology. To build a comfortable, safe driving environment and improve the landscape and ecology along highway, The theory and strategy of a aesthetics characteras of tree belts such as shape and color, traffic safety, biodiversity, slope stability, environmental protection and pest control were applied to the design of tree belt along highway.