Random-Incidence Absorption and Scattering Coefficients of Vegetation

Sound absorbing and scattering properties of vegetation become more important as it is increasingly grown on building façades and the ground in urban spaces. Vegetation consists of mainly two components including the growing media (soil) and the plant (leaf, stem and root), which play different roles in absorbing and scattering sound. Therefore, it is necessary to characterise systematically how each or combined components become effective. In this study, a series of measurements were carried out in a reverberation chamber to examine random-incidence absorption and scattering coefficients of vegetation, by considering various factors such as soil depth, soil moisture content and the level of vegetation coverage. The results for different soil depths (50, 100, 150, 200 mm) showed that even the thin soil layer with a depth of 50 mm provided a significant absorption coefficient of about 0.9 at around 1000 Hz and there were only slight changes of absorption coefficient of about 0.1 with increased soil depth. A significant decrease by about 0.6 in absorption coefficient was observed with the increase in soil moisture content. With increasing vegetation coverage, the absorption coefficient increased by about 0.2 at low and mid frequencies, whereas over about 2000 Hz the absorption coefficient was slightly decreased by about 0.1. It was shown that the stronger effect on sound absorption and scattering by aboveground vegetation components (excluding roots and soil) was found at higher frequencies with increasing vegetation coverage. The maximum absorption and scattering coefficients of aboveground vegetation studied here was 0.49 at 5000 Hz and 0.43 at 2500 Hz, respectively. It was also found that a green wall with highly porous substrate kept a relatively high absorption coefficient of about 0.6 even though it was nearly saturated.