Evaluation of Noise Absorption by Dodonaea viscosa L. Plant as a Common Green Wall in Ahwaz City

Population growth and increased urban migration in the past decades have led to an increase in population density and size of major cities. Unfortunately, this kind of pollution has mostly gone unnoticed. To prevent these effects and achieve recommended standards, the use of plants as noise barriers in cities is mandatory. Trees offer a range of ecosystem services and remain important in providing human benefits. The aerodynamic effect of trees was identified as a major reason for the change of pollutant distribution in near-road parks, where trees can act as porous barriers and decrease noise pollution. Although not yet fully developed, planting strategies aiming to benefit from the negative effects of vegetation on noise pollution should be encouraged in future park design.

The present study was conducted to investigate the absorption of noise pollution by one of the plants in the landscape of Ahvaz, Dodonaea viscosa L., in Agricultural Sciences and Natural Resources University of Khuzestan. The experiment was conducted as a completely randomized design with different distances between the sound source and the green wall (5, 10, and 15 m) and green wall width (0.5, 1, 1.5, and 0 m as control). After receiving the sound by the receiver, the data were transferred to MATLAB (2015) software. Then, the properties of the sound level were measured including minimum, maximum, elongation, skewness, mean, and variance.

The results showed that with increasing the distance from the sound source, the minimum sound intensity decreased and finally, the green wall with one-meter width at 15-meters distance had the minimum intensity (68.4 dB). It was also found that the maximum sound decreased with increasing distance of the sound source from the green wall so that the maximum was related to the distance of 5 meters (91.6 dB) and the lowest was related to the distance of 15 meters (76.3 dB). Also, the analysis of the data showed that in the control and other thicknesses of the green wall, the average sound intensity decreased with increasing distance from the sound source. The highest variance of sound intensity was found in 1 m width green wall and 5 m away from the sound source and the lowest variance of sound intensity was related to the thickness of the control at a distance of 5 meters from the sound source.

Basically, the sound waves are absorbed by the leaves and branches of trees. The higher the density of the tree and the shorter the distance from the sound source to the tree, the greater the reducing effect. With the shorter distance from the green wall, the sound gradually decreases because it creates an angle at the deflection path of the wave and therefore, the wave has to travel a greater distance to the receiver. According to the obtained results, it is suggested for landscape designers to use this shrub as a green wall and sound barrier, pruning it with a width of one meter and also its distance from a sound source close to about 5 meters.