MEASUREMENT OF ROAD TRAFFIC NOISE ATTENUATION DUE TO TREE GROVES

Road traffic noise attenuation due to roadside greenbelt can be estimated from existing knowledge on attenuation due to the ground. However, attenuation due to tree groves—described here as tree attenuation (Awoods)—in greenbelt cannot be estimated, as well-established knowledge on Awoods is not available. There are two difficulties in obtaining accurate knowledge. First, many of the Awoods measured in previous research include attenuation due to the ground etc. Second, the scales differ depending on the measurements. Taking the difficulties into account, well-planned measurements and analysis at a park and three road sides have been carried out in this paper. The measurements at the park were planned to exclude attenuation from diffraction or the ground. Pink noise emitted by a nondirectional semi-spherical speeker was measured and converted to average spectra data of road traffic noise. However, to exclude entirely such attenuation for roadside measurements is impossible. Thus, the equivalent continuous A-weighted sound pressure levels measured at the roadsides, LAeq, were analyzed by calculating and excluding some attenuation resulting from diffraction around the greenbelt, the ground, and atmospheric absorption. The results are as follows: Awoods estimated by the test sounds was -0.23 dB/m and -0.17 dB/m. Awoods estimated by the LAeq measured at the roadsides was -0.12 dB/m, -0.10 dB/m, and -0.04 dB/m. The Awoods measured in the previous research were also converted to the same scale values by calculating and excluding some attenuation due to the ground etc. The Awoods in this paper were intermediate values in comparison with plural references. As the data is not sufficient, the values are not certain, but taking account of noise reduction by tree groves in greenbelt will be practical in the future if a sufficient number of measurements can be carried out.