than that at 100 Hz by approximatel

than that at 100 Hz by approximately 15 dB(A). The effect of thebarrier on high-frequency noises was better than that on lowfrequency noises and the superiority weakened with distance.4. Parametric analysis4.1. Effect of barrier heightThe region behind the barrier can be divided into acoustic shadow and low-frequency, mid-frequency, and high-frequency cutoff zones from the bottom upward (Fig. 16), and this distributionchanges depending on the structure. Because the high-frequencysound has a faster rate of decay and is susceptible to the heightof the noise barrier, increasing the height of the barrier effectivelyenhances the noise reduction ability.By increasing the height from 2 m to 6.5 m with a gradient of0.5 m, the noises at nine field points were predicted (Fig. 17). Asshown in Fig. 18, the insertion loss has a positive correlation withthe height of the barrier. The effects were greater at lower pointsand were weakened with the increasing horizontal distance.The ‘‘height effect”, i.e., the increment of insertion loss in each0.5 m–increasing height, is used to evaluate the influence of height.As illustrated in Fig. 19, the maximum enhancement can beobtained by heightening the barrier in the range of 3–4 m. Theincrement became smaller when the barrier was heightened, andthis was involved by the positions in the sound field to someextent. Therefore, the rise in the height of the barrier should bethe result of overall consideration of various factors.The height of the barrier has an impact on the insertion loss ofthe barrier in the full frequency range to different extents and is