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  • Smyrnovaa and Yuliya conducted a set of experimental measure

    2018-11-12

    Smyrnovaa and Yuliya (2010) conducted a set of experimental measurements inside a reverberation chamber using a probe tube for two samples of Buxuc plants from the Buxaceae family. The results for the two set of measurements are shown in Figure 9. According to [ASTM C423], the Sound Absorption Average (SAA) of the 1/3 octave sound EZLink Sulfo-NHS-LC-Biotin Kit coefficient values in the diffuse field of the plants corresponds to the arithmetic average, , for the twelve 1/3 octave frequency bands ranging from 200Hz to 2500Hz and is calculated according to the following equation: Given the complexity associated with the geometrical modeling of the urban texture, SAA is used as a single figure to account for all frequency bands in the computer model. This condition implies several precautions, given that the audible frequencies are not all accounted for in the SAA and that the third octave bands around 4000Hz are not used in its computation. However, considering the complexity of the computer model, this assumption had to be considered. The SAA values for the two sets investigated by the previous researchers are shown in Figure 10. To measure the absorption coefficients of streets, Steenackers et al. (1965) observed a variation in the measured absorption of façades, with the absorption coefficient ranging from 0.15 to 0.35 as a function of street width. They suggested that the large sound energy absorption measured in wide streets can be attributed to the scattering of sound out of the street by trees and other objects. Thus, the current study selected a 0.2 order of absorption for the street façades because of the additional acoustic energy losses attributed to scattering outside the street, which are accounted for by the energy transfer equations and mechanism used in the model. The vertical greenery erected on street façades presents various designs, as shown in Figure 11, which implies a variation in absorption characteristics according to the species of plantation used. The value of the absorption coefficient for the variety of plants investigated ranges from 0.41 to 0.99. This absorption coefficient range will be further analyzed via computer model simulation.
    Computer model simulation Based on the given criteria, the model is used to simulate a 410m×410m urban area, with six lateral and six perpendicular streets. The building density is set as 73%. With this setup, the urban pattern will hold the same geometric configurations as the Islamic urban parameters discussed earlier. The four absorption coefficients of the green vertical façades are 0.41, 0.48, 0.54, and 0.85. Figs. 12,13,14 show building heights of 4, 8, and 12m, respectively. The figures show contour plots along the diagonal attenuation curves from the source position located at the bottom left corner of the modeled urban square area to the upper right corner. The data plotted are represented in relative attenuation to the absorption of regular street façades measured by Steenackers et al. (1965), thus representing additional attenuations attributed to the installation of vegetation on the vertical street façades. All plotted data clearly show that vertical green façades introduce a significant amount of additional attenuation relatively near the source location. The relative attenuation tends to have a constant value for long distances from the source because propagation losses are set off by the increased energy from more façades at remote locations from the source. The relative attenuation should be independent of and only marginally affected by the source position because in a real situation, the line of sight will only exist in areas very close to the source. However, with the regular array of buildings, receiving direct sound is only possible if the source and receiver are located along the same street. The results also show that the resultant attenuation at remote locations from the source is slightly affected by an increase in the building height. The additional attenuation resulting from the increase in building height from 4m to 12m was only 1dB, which could be considered marginal. However, in this case of height increase, the surface area of the vertical green wall is larger by a factor of 3.