Performance of acoustic noise barriers in the Sayyad Shirazi highway in Tehran

Introduction:
Today, noise pollution is recognized as serious health and environmental problem. With the spread of urbanization and migration and the construction of roads, rails, and air transport networks, as well as the rapid development of the industry over time, the issue of noise pollution is considered a serious issue that prompted urban planning officials to provide a suitable solution for it. The use of sound barriers is one of the methods that is used in reducing and controlling the sound propagation path and is effective in reducing the traffic noise due to the movement of tires on the road, air passing rapidly over the vehicle, vehicle traffic, and some intentional anomalies (such as raising the volume of internal speakers, tampering with car exhaust, etc.). Currently, 20 highways in Tehran have noise pollution, and in some cases, sound barriers have been installed. The Sayyad Shirazi Highway is also equipped with three sound walls. The purpose of this study was to investigate the performance of the sound barriers located in the Sayyad Shirazi Highway.
Material and methods:
In this study, a three-section noise barrier was installed in Sayyad Shirazi Highway using a B&K2236 sound level meter according to the standard of the US-EPA in terms of sound level (at a height of 1.5 meters from the ground) in different positions relative to the wall. The sound level was measured at a distance of 5 and 20 meters behind the wall and then two factors of sound level reduction and the amount of sound loss were evaluated. Also, by measuring the sound level at houses (with a stone facade and double glazing) and in the living room of different floors, the equivalent sound level was compared with the sound standard in houses compiled by the US-EPA.
Results and discussion:
The equivalent sound level inside the highway varied between 70 to 80 dBs, and at a close distance (5 meters) behind the barrier, a decrease of about 15 dBs, and at a distance of 20 meters, a decrease of about 12 dBs was observed. Although in the barrier of Zanbagh Alley, due to the short length of the barrier and not properly designed barrier, the sound barrier (lack of proper installaion angle) was lower. Nevertheless of the reduction of about 12 dB, the equivalent sound level does not comply with standard limit (55 dB). It is difficult to meet the national standard due to the short distance between the highway and residential houses., Inside the houses behind barriers, measurements also indicated a relative non-fulfillment of standards. Although it seemed that with increasing altitude, the effect of sound barriers in reducing the equivalent level of sound would be less, this was not observed until the third floor, and sometimes even due to the loud noise of the utility room, the noise level in the first floor, was higher than the second and third floors. Due to the 6 meters height of the wall, with increasing height and in the fourth and fifth floors of houses behind the noise barrier, the effect was less, however, buildings with more than three floors were less observed near the barrier and the highway.
Conclusion:
The performance of the noise barrier was generally positive in reducing the equivalent sound level. The sound barriers of Bustan Vahed between Farvardin and Abuzar and the noise barrier of Zanbagh Alley were able to significantly reduce the noise level caused by traffic to a lesser extent. According to this study, none of the locations complied with the national standard limit (55 dB) of noise pollution.