Noise Pollution Reduction In Relationship With Urban Forestry And Urban Greening
The main objective of this paper is to understand and examine various noise pollution reduction and attenuation methods. Noise modifies social behavior, endangers complex task performance, reduces productivity and causes serious health issues like sleeplessness, deafness and mental breakdown. Noise pollution is adversely affecting millions of individuals as it is damaging physiological and psychological health. According to the World Health Organization, noise pollution is the third most hazardous environmental type of pollution after air and water pollution. Roadside vegetation which has the potential to act as noise barriers in urban enivronment has been examined indepth with both Indian and foreign context.
Noise is derived from the Latin word “nausea” implying ‘unwanted sound’ or ‘sound that is loud, unpleasant or unexpected’. The noise originates from human activities, especially the urbanization and the development of transport and industry. Though, the urban population is much more affected by such pollution, however, small town/villages along side roads or industries are also victim of this problem. Noise is becoming an increasingly omnipresent, yet unnoticed form of pollution even in developed countries. According to Birgitta and Lindvall (1995), road traffic, jet planes, garbage trucks, construction equip-ment, manufacturing processes, and lawn mowers are some of the major sources of unwanted sounds that are routinely broadcasted into the air.
Though noise pollution is a slow and subtle killer, yet very little efforts have been made to ameliorate the same. It is, along with other types of pollution has become a hazard to quality of life. Kiernan (1997) finds that even relatively low levels of noise affects human health adversely. It may cause hypertension, disrupt sleep and/or hinder cognitive development in children. The effects of excessive noise could be so severe that either there is a permanent loss of memory or a psychiatric disorder (Bond, 1996). Thus, there are many an adverse effects of excessive noise or sudden exposure to noise.
In India, the problem of noise pollution is wide spread. Several studies report that noise level in metropolitan cities exceeds specified standard limits. It is responsible for rising incidence of deafness among the inhabitants (Bhargawa, 2001). A study by Singh and Mahajan (1990) conducted in Delhi and Calcutta, found that the noise level is 95dB as against the ambient limit of 45dB. Even at the “calm” places, it does not fall below 60dB. Murli and Murthy (1983) also found that traffic noise in Vishakhapatanam exceeds 90dB even in morning hours that acts as a source of nuisance. The noise pollution is not a unique problem for developing countries like India only. In China, till third century B.C., instead of hanging men for dangerous crimes, noise was used for their torturing. The worrisome effects of noise are dangerous enough that noise problem is consi-dered next to crime by certain countries (Kapoor and Singh, 1995). Bond, (1996) reports that 16% of people in Europe are exposed to 40 dB or more of traffic noise in their bedrooms at night compare it with W.H.O.’s average estimates of 30 to 35 dB for undisrupted sleep.
The acoustic performance of vegetation in reducing noise
The development of green belt around the noise polluting area is a useful measure in order to reduce noise pollution level (Aylor, 1972, Tyagi, 2006). Different researchers have already studied the efficiency of green belt in the mitigation of noise pollution (Rao et al., 2004, Krag, 1979, Cook, 1977). The main mechanism associated in the mitigation of noise level by the plants includes the absorption of acoustic energy by its leaves which reduces the kinetic energy of the vibrating air molecules (Pathak, 2011). Another useful mechanism adopted by the plants in reducing sound pressure level is the barrier effect of green plants which causes greater reflection, refraction, scattering and absorption of noise resulting in higher amount of attenuation of sound (Fang and Ling, 2003). Some important design parameters considered in the development of green belt to reduce noise pollution level includes trees height, width of green belt, distance of green belt from the source, visibility etc. The vegetation species selected for such function are required to have fast growth rate for quick development of a canopy, large leaf size for enhanced retention of pollutants and absorption of noise. These plants are also required to have comparatively good tolerance to insects, diseases and other pollution loads. Longer life span is also an essential requirement to ensure the longevity of the developed green belt (Shannigrahi, 2004). As per the study made by some researchers for highway noise management through green belt development, it indicates that an effective highway noise absorption and deflection can be done if the border planting are lower towards the noise and higher towards the hearer (Pathak, 2011, Rao, 2004).
The study by Mengmeng Li and Jian Kang (2018) investigated the vibration of leaves of 6 plant species in a sound field using a Keyence (IG- 1000 /CCD) Laser Micrometre. The results showed that the vibration amplitudes of plant leaves increase significantly by about 4-12 μm after being stimulated by sound. In addition, driven by the same sound, the amplitudes of all leaves varied with the difference of leaf thickness, leaf area and leaf mass. The amplitudes of all leaves increase with the increase of leaf area and leaf mass, while decrease with the increase of leaf thickness.
The research article titled Effect of roadside vegetation on the reduction of traffic noise levels by Kalansuriya, Pannila and Sonnadara (2009) has showed that higher frequency noise (above 4 kHz) is heavily attenuated by the vegetation barriers with virtually no attenuation for low frequency noise (below 100 Hz). The width of the vegetation barrier is linearly proportional to the amount of sound absorption. Without the vegetation barrier, the observed maximum and minimum noise levels were 72 dB(A) and 64 dB(A) respectively. On average, vegetation barriers were able to reduce the noise by 4 dB(A) which corresponds to an approximately 40% acoustic energy reduction. Thus, with careful planning and growing of roadside vegetation, the effect of road noise can be reduced.
The research article titled Importance of the Green Belts to Reduce Noise Pollution and Determination of Roadside Noise Reduction Effectiveness of Bushes in Konya, Turkey by Onder and Kocbeker has done impact analysis of vegetation as noise blockers on highway of Konya city (Turkey). The biggest value of noise reduction was 6.3 dB(A), 4.9 dB(A), 6.2 dB(A) value with compared to the control which includes the group that formed by the bushes at the distance of 7m, 11m, 20m from the source and 5m, 9m, 20m of plant width, respectively. The plant groups which were consisted from Pyracantha coccinea M., Cotoneaster horizontalis Decne., Berberis thunbergii D.C., Cotoneaster dammeri C.K., Forsythia intermedia Zab., Juniperus horizontalis L., Spirea vanhouetti Briot., Tamarix tetranda L., Euonymus japonica L. were used in a width of 5m, 9m and 20m to determination of decreasing effect on the noise in Konya.
The research article titled Road Traffic Noise Attenuation by Vegetation Belts at Some Sites in the Tarai Region of India by Vikrant Tyagi, Krishan Kumar and V.K Jain (2013) has assessed the effectiveness of vegetation in reducing road traffic noise levels in the Indian context by obtaining the sound attenuation spectrum of vegetation at eleven sites located in Dehradun, Pantnagar, and Haridwar, which are some of the prominent cities of the Tarai region in India.
Vertical greenery systems
Vertical Greenery Systems (VGS) are vegetated structures conceived to be installed on building facades.  Acoustic insulation capacity of Vertical Greenery Systems for buildings by G. Pérez et al provided me with detailed study of acoustic insulation capacity of two different VGS, double-skin green facade (extensive system) and living wall or green wall (intensive system).
The main results obtained after comparing low and high values of vegetation, highlighted the differences between the double-skin green facade (GF) and green wall (GW) systems. The importance of substrate contribution to noise attenuation in the GW allowed developing a constant noise profile along the frequency spectrum tested in both measured periods, whereas GF showed a much more irregular profile. In addition, the improvement of the acoustic insulation capacity from both greenery systems provided by plants (scattering) in high frequencies, as well as from substrate (absorption) in the middle frequencies by Green Wall, were verified in the standardized difference of levels profiles.
I conclude that further research is required at large scale to quantify accurately which plant species is suitable for noise attenuation and reduction effectively and to identify various implementaion issues regarding Vertical Greening System (VGS) in urban enivronment. Trees with dense foliage i.e those with rich canopy were found to be highly effective in absorbing the acoustic noise and act as very good screens in bringing down the noise levels. Mixing species of trees having a larger quantity of leaves in high density stands may improve the efficiency of forest stands for noise attenuation in urban area. Foresters must be involved in our endeavour to understand the interaction between the trees and the urban environment for a noise pollution free ecosystem. The vegetation barriers for noise attenuation also offer an aesthetic element for environmental integration.
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