A main way for sound reduction by tree belts is to scatter sound waves by trunks while previous studies of sound absorption by trunks always contain the ground effects. This study aims to examine the difference of reverberation time in different kinds of tree belts through eliminating the influence from ground effect. In order to estimate the scattering of sound waves by trunks, this study investigates sound (Impulse sound) attenuation by three woodlands with same tree species and one woodland with mixed species in the ground with thick snow. The effect of receiver distance from the sound source was also investigated (5 m,10 m and 20m). The results showed that reverberation time can be used to describe the sound propagation over snow cover through tree belts. The reverberation time in different kinds of forests is different, but the variation shows a similar trend with the increase of frequency. The distance between the receiver and sound source affects the reverberation time. The species with higher ability of sound scattering can be used in urban green space to reduce urban noise

Kang, Jian

Li, Mengmeng

Meng, Qi

English

UCL Discovery

 PROCEEDINGS of the  24th International Congress on Acoustics   October 24 to 28, 2022 in Gyeongju, Korea ABS-0152   SOUOND ATTENUATION BY TRUNKS IN THE GROUND WITH THICK SNOW  Mengmeng LI1,2; Qi MENG1,2; Jian KANG3 1 School of Architecture, Harbin Institute of Technology, Harbin 150000, China 2 Key Laboratory of Cold Region Urban and Rural Human Settlement Environment Science and Technology, Ministry of Industry and Information Technology, Harbin Institute of Technology, Harbin 150000, China 3 Institute for Environmental Design and Engineering, The Bartlett, University College of London, London WC1H 0NN, UK ABSTRACT A main way for sound reduction by tree belts is to scatter sound waves by trunks while previous studies of sound absorption by trunks always contain the ground effects. This study aims to examine the difference of reverberation time in different kinds of tree belts through eliminating the influence from ground effect.  In order to estimate the scattering of sound waves by trunks, this study investigates sound (Impulse sound) attenuation by three woodlands with same tree species and one woodland with mixed species in the ground with thick snow. The effect of receiver distance from the sound source was also investigated (5 m,10 m and 20m). The results showed that reverberation time can be used to describe the sound propagation over snow cover through tree belts. The reverberation time in different kinds of forests is different, but the variation shows a similar trend with the increase of frequency. The distance between the receiver and sound source affects the reverberation time. The species with higher ability of sound scattering can be used in urban green space to reduce urban noise.  Keywords: Sound propagation, Reverberation time, Tree belts, Ground effect 1. INTRODUCTION Sound propagation outdoor is obviously affected by the surrounding environment. Compared with open ground, the effect from tree belts on sound propagation is complex, which includes ground effect, sound scattering by trunks and leaves, etc.   For the short distance propagation of sound waves in tree belts, multiple scattering is an important influential factor. Wunderli found that the sound reflection from tree belts mainly occurs in the fringe[1]. Some studies found that the sound attenuation effect is affected by the physical properties (length, width, height, etc.) of tree belts[2-5]. Further studies explored the influence from species, leaf shape, bark roughness[6-9]. Karbalaei found that the noise reduction effect of arbor shrub mixed forest land and coniferous broad-leaved mixed forest land is relatively large, by studying the influence of tree belt width and plant species diversity on the noise reduction effect [10]. For tree belts with similar physical properties, the influence from other factors including plant species and tree age on the noise reduction effect is important[11-12]. However, ground effect is always incorporated in the process of field investigation. Furthermore, Aylor found that the influence from ground effect is inevitably involved in the noise reduction by tree belts in outdoor[13]. By studying the sound propagation over the snow cover, Sidler found that the sound attenuation effect decreases with the increase of snow surface depth [14]. Albert further demonstrated that in the snow, cover parameter associated with snow surface shows dominant impact, by studying low frequency pulse sound propagation in the forest [15]. Therefore, the test on the next day of heavy snow can reduce the ground effect on sound propagation through tree belts.   1 mengmeng.li@hit.edu.cn 2  mengq@hit.edu.cn 3  j.kang@ucl.ac.uk   Yang et al. found that reverberation time is applicable in the measurement of the sound scattering effect of a single tree in the open ground [16]. Reverberation time is further authenticated to be applicable in the forest[17]. This study aims to estimate the scattering of sound waves by trunks, by investigating reverberation time by three woodlands with same tree species and one woodland with mixed species in the ground with thick snow. In this research, the influences from plant species and the distance between the sound source and receiver on the reverberation time are specified.  The findings can be used to guide the urban tree belt design.  2. METHODOLOGY 2.1 Site Condition The site in this study should be a forest with no less than 40m in length and 25m in width. The test site is 121 ° 12 ′ to 127 ° 00 ′ E, 50 ° 10 ′ to 53 ° 33 ′ N, and the average altitude is 573m, located in the Northeast China. The temperature condition in test is -24℃, and the wind speed is less than 2m/s. The test is carried out on the day after a heavy snow.  The photo taken in the test is shown in Figure 1, which are Pinus sylvestris seedling, Birch, Shrub, and Forest Mixed with Conifer and Broadleaves, respectively. The parameters representing the trees in the test site are shown in Table 1. Table 1 – The parameters representing the trees in the test site Plant species Average Breast-height Diameter, cm Average height, m Pinus sylvestris seedling 31.23 3.3 Birch 25.19 6.5 Shrub 19.43 2.8 Mixed with Conifer and Broadleaves 70.57/13.4 11.2/5.5    Pinus sylvestris seedling Birch   Shrub Forest mixed with conifer and broadleaves Figure 1 – Photos of the testing site 2.2 Measurement method The measuring point arrangement is shown in Figure 2. This research measured the reverberation time of the microphone when the height is 1.5m, and the distance with the sound source are 5m, 10m and 20m, separately. The measurement of all settings of distance repeated for 5 times, and the average value is used for analysis.    Figure 2 – Schematic diagram of measuring point arrangement Because measuring T60 needs a high signal to noise ratio, which is difficult to implement outdoor, T60 is not measured in this study. T10, T20 and T30 are widely used for outdoor sound propagation, and this study chooses T20 as the index of reverberation time. The site in this study is far away from urban area and roads, so it is relatively quiet. The SPL of the gun is about 55dB h igher than the background noise, so it is feasible to measure T20 under this the signal to noise ratio. The software ‘DRIAC’ is used to analyze the data. 3. RESULTS 3.1 The variation trend of T20，T20 and T30 Figure 3 shows the variation of T10, T20 and T30 of Birch and Shrub when the distance is 20m. Though the reverberation time of the two sites is different, the trend of variation with the increase of frequency is identical, which complies previous findings. The reverberation time measured over the snow cover can validate that the software ‘DRIAC’ is precise enough in the analysis. In the following part of this study, T20 is selected to analyze sound propagation over snow cover through tree belts.   Figure 3 – T10, T20 and T30 of Birch and Shrub when the distance between receiver and sound source is 20m 3.2 The influence from species on T20 Figure 4 shows the spectral characteristics of T20 in site of different plant species. The T20 of all kinds of tree belts share a similar trend of variation with the frequency. When the frequency is below 200Hz, the T20 of all kinds of tree belts are relatively low. For the condition of the frequency between 200Hz and 400Hz, the T20 of all the tree belts increase significantly with the increase of frequency. When the frequency is above 400Hz, the relationship between the frequency and T20 of Pinus sylvestris seedlings and Shrub reaches to a stable trend, and the T20 of Forest mixed with conifer and broadleaves and Birch decreases slightly with the increase of frequency.      Figure 4 – Spectral characteristics of T20 in site of different plant species The T20 of Forest mixed with conifer and broadleaves and Birch are largest , which can be caused by the large age, bark roughness and Breast-height Diameter of these two kinds of tree belts. The T20 of Pinus sylvestris seedlings is larger than Shrub. This phenomenon can be caused by two reasons. First, the trunks of Shrub are small, and cannot insulate sound effectively. Second, the barks of Shrub are smooth, which leads to a lower sound absorption coefficient than Pinus sylvestris seedlings. 3.3 The influence from distance on T20 The spectral characteristics of T20 of Forest mixed with conifer and broadleaves with different kinds of distance between receiver and sound source is shown in Figure 5. The T20 of different kinds of distance show a similar trend of variation with the increase of frequency. When the distance increase form 5m to 10m, the change in T20 is negligible.T20 shows an obvious increasing trend, when the distance increases from 10m to 20m.    Figure 5 – Spectral characteristics of T20 of Forest mixed with conifer and broadleaves with different kinds of distance between receiver and sound source  4. CONCLUSIONS This research found that the reverberation time of sound propagation over snow cover through different kinds of tree belts is different, but the reverberation time shows a similar variation trend with the increase of frequency.  When the plant species is different, the reverberation time varies. In this study, 4 plant species are tested, and Birch and Forest mixed with conifer an broadleaves has the longest reverberation time.   The planting density of Birch is obviously larger than Forest mixed with conifer an broadleaves, which can demonstrate that forest consisted of trees with smoother barks needs larger planting density in order to attain a similar reverberation time to forest consisted of trees with rough barks. The reverberation time of Pinus sylvestris seedlings and Shrub is less, and Pinus sylvestris seedlings has a longer reverberation time than Shrub, which is caused by the fact that Pinus sylvestris seedlings has trunk to insulate sound. Moreover, the distance between receiver and sound source can affect the reverberation time, and the reverberation time reach the peak when the distance is 20m in this study.  Plant species, planting density and bark roughness affect the sound propagation outdoor, but the effect is not quantified. Further studies can measure the sound propagation in more kinds of forests and find the dominant influential factors.  REFERENCES 1. Wunderli J M, Salomons E M. A model to predict the sound reflection from forests[J]. Acta Acustica United with Acustica, 2009, 95(1):76-85. 2. Fang C F, Ling D L. Investigation of the noise reduction provided by tree belts[J]. Landscape & Urban Planning, 2003, 63(4):187-195. 3. Pathak V, Tripathi B D, Mishra V K. Dynamics of traffic noise in a tropical city Varanasi and its abatement through vegetation[J]. Environmental Monitoring & Assessment, 2008, 146(1-3):67-75. 4. Maleki K, Hosseini S M, Nasiri P. The Effect of Pure and Mixed Plantations of Robinia Pseudoacasia and Pinus Eldarica on Traffic Noise Decrease. international journal of environmental sciences, 2010. 5. Lai F O, Ghosh S. Urban cities and road traffic noise: Reduction through vegetation[J]. Applied Acoustics, 2017, 120:15-20. 6. Fan Y, Zhiyi B, Zhujun Z, et al. The investigation of noise attenuation by plants and the corresponding noise-reducing spectrum[J]. Journal of Environmental Health, 2010, 72(8):8. 7. Reethof G. Effect of Plantings on Radiation of Highway Noise[J]. Journal of the Air Pollution Control Association, 1973, 23(3):185-189. 8. Reethof G, Mcdaniel OH, Heisler GM. Sound absorption characteristics of tree bark and forest floor. Usda Forest Service General Technical Report Ne.1977. 9. Li M, Van Renterghem T, Kang J, et al. Sound absorption by tree bark[J]. Applied Acoustics, 2020, 165:107328. 10. Karbalaei S S, Karimi E, Naji H R, et al. Investigation of the Traffic Noise Attenuation Provided by Roadside Green Belts[J]. FLUCT NOISE LETT, 2015, 2015,14(4)(-):-. 11. Bashir I, Taherzadeh S, Shin H C, et al. Sound propagation over soft ground without and with crops and potential for surface transport noise attenuation[J]. The Journal of the Acoustical Society of America, 2015, 137(1):154-164. 12. Van Renterghem T, D Botteldooren, Verheyen K. Road traffic noise shielding by vegetation belts of limited depth[J]. Journal of Sound and Vibration, 2012, 331(10):2404-2425. 13. Aylor D. Noise Reduction by Vegetation and Ground[J]. J. acoust. soc. am, 1972, 51(1B):197-205. 14. Sidler R. A porosity-based Biot model for acoustic waves in snow[J]. Journal of Glaciology, 2015, 61(228):789-798. 15. Albert D G, Swearingen M E, Perron F E, et al. Low frequency acoustic pulse propagation in temperate forests[J]. Journal of the Acoustical Society of America, 2015, 138(2):735-47. 16. Yang H S, Kang J, Cheal C, et al. 2013. Quantifying scattered sound energy from a single tree by means of reverberation time. Journal of the Acoustical Society of America 134, 264. 17. Albert D G. Past Research on Sound Propagation through Forests[J]. Past Research on Sound Propagation Through Forests, 2004. 

Sound Attenuation by Trunks in the Ground with thick Snow

https://discovery.ucl.ac.uk/id/eprint/10172662/1/ICA_2022_template_final_ABS-0152.pdf

Sound Attenuation by Trunks in the Ground with thick Snow

Abstract

Similar works

Full text

Available Versions

UCL Discovery