The Principle And Influencing Factors Of Noise Reduction By Plants

With the rapid development of cities,noise pollution is becoming an increasingly serious problem.Many laws,regulations and rules relate d to noise reduction have been issued worldwide to control the impact of noise on people’s well-being.Urban landscape composed of a variety of green plants is an important part of cities to improve the urban ecology and the urban environment,and its effe ct on noise control is also quite obvious.As the area available for urban green space is shrinking,it is of great practical significance to simply and efficiently use the limited plant space to reduce more noise.At present,the research on noise attenuation by using urban green space has been quite deep,but there is still a lack of theory and application research on the noise reduction principle of different plant structures.What’s more,the principle of noise reduction by plants has not been systematically applied to urban noise control.So the aim of this study is to study the principle of noise attenuation by single tree and tree belts through the combination of experiment and actual measurement.In addition,in order to build the prediction models of noise reduction by leaf vibration and sound absorption by tree bark respectively through explore the influence of plant morphological characteristic parameters on plant noise reduction effect.Firstly,focus on the single leaf,the vibration characteristics of plant leaf in the sound field are explored in this study.It is found that in the same sound field,the noise reduction effect of plant leaf is stronger than that of cardboard,PVC sheet and COMSOL simulation with the same morphological characteristics at the same vibration speed.In addition,the difference of leaf response sound can be found in different frequency,and the frequency of noise reduction by leaf vibration is mainly concentrated in the range of 2-4KHz.For the same leaf,there are significant difference in vibration at different positions of leaf.The vibration velocity at the vein is slightly slower than that at the mesophyll,and the vibration velocity at the edge of the leaf is higher than that at the center of the leaf.When the vib ration velocity of the leaf is less than 0.6mms^-1,the vibration velocity is positively correlated with the attenuation of noise,that is,the noise reduction effect by leaf increases with the increase of leaf vibration velocity.Secondly,Secondly,focus on leaf morphological characteristics,this paper explores the influence of leaf morphological characteristics on the leaf vibration after being stimulated by sound.It was found that the most important factor affecting the vibration condition of leaves stimulated by sound is leaf texture.For plant leaf with the same texture,leaf area,thickness,mass,petiole length,petiole width and petiole thickness are the main factors affecting noise reduction by vibration.Compared with succulent leaf,the vibration amplitude and velocity of leather,papery and membranous leaf increased significantly after sound stimulation.For plant leaf with the same texture,leaf area,thickness,mass,petiole length,petiole width and petiole thickness are the main factors affecting noise reduction by vibration.Without sound stimulation,the leaf vibration amplitude（～1μm）and speed(～0.05mms^-1)are relatively small,while the leaf vibration amplitude（～4-12μm）and speed(～0.5-1mms^-1)are significantly improved with the sound stimulation.Finally,in order to quantify the influence of leaf morphological characteristics on noise reduction effect by leaf,a prediction model of vibration of leathery leaf under sound stimulation based on leaf mass,area,petiole thickness,petiole length and petiole width was constructed.In addition,a multiple linear regression model for predicting leaf vibration velocity under sound stimulation based on leaf mass and thickness is constructed.Thirdly,focus on tree bark,this study explores the sound absorption characteristics of bark of different plant species.It is found that the sound absorption coefficient of bark significant differences between different plant species significantly,while there is no difference of bark absorption coefficient can be found between different bark samples of the same species.Moss on the bark surface can significantly improve the sound absorption coefficient of bark in the low frequency range.With the increase of frequency,the sound absorption coefficient of bar k samples increases,but still lower than 0.1.In terms of bark sound absorption coefficient,tree age and bark sample roughness（R4）are the most inportant influencing factors.Through full wave simulation to quantify the sound absorption effect of bark,it is found that even if the sound absorption coefficient of bark is only slightly improved（0.02）,the noise reduction effect of plant forest belt can be better improved by about 10%.Finally,focus on tree belts,this study explores the important of acou stic characteristics of leaf and bark in noise reduction by tree belts.First,it is found that the reverberation time（T30）is significantly different between different tree belts through the testing of sound attenuation of the tree belts in the snow,but they are clearly higher than that in the blank snow.With the increasing of the distance between sound source and receiving point,the reverberation time of tree belts increases gradually.When the distance between sound source and receiving point increases from 0m to 5m,the reverberation time of tree belts increases significantly,while the distance between sound source and receiving point increases from 5m to20m,the reverberation time of tree belts increases slightly.The early attenuation time of tree belts is higher than that of blank snow,in addition,an clear difference can be found in different tree belts.Under 4HKz,with the increase of frequency,the early attenuation time of tree belts increases,while the fluctuates can not be found in early attenuation time of tree belts with the frequency increase when the frequency is higher than 4HKz.The reverberation time of tree belts is different between different species,for this study,the reverberation time of tree belts is similar among coniferous,coniferous broad-leaved and broad-leaved tree belts,which is higher than that of shrub and small mongolica tree belts.Then,it is found that the noise reduction effect of equal width plant forest belt with different planting methods was significantly different through the testing of noise reduction effect of equal width tree belts with different plant configurations.In addition,the noise reduction effect by tree belts is insignificantly and fluctuated when the frequency is less then 200Hz,while the noise reduction effect by tree belts is relatively stable when the frequency is between 200Hz and 2000Hz,what’s more,at higher frequency（more than 2000Hz）,the noise reduction effect of plant forest belt increases with the increase of frequency.For noise reduction effect by tree belts,the most important effect factor is the configuration in the vertical direction and leaf density.