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degree. Refraction, on the other hand, can exert a strong influence on received highway traffic noise levels. Refrac- tion can be caused by wind shear, which is the change in wind speed with increased height above ground as well as by temperature lapse rate, which is the change in tempera- ture with increased height. A neutral atmosphere with no re- fraction occurs on a calm cloudy day. For wind shear under upwind conditions (the receiver is upwind from highway), sound levels are quieter near the ground than under neutral conditions. Under downwind conditions, sound levels are louder near the ground. For temperature lapse rate, upward refraction occurs on sunny, calm days, when it is warmer near the ground. Under these conditions, it is quieter near the ground than on a cloudy day. Downward refraction oc- curs on calm cool nights after sunny days when the ground cools off faster than the air above, and under these condi- tions, sound is louder near the ground (National Highway Institute [NHI], 2016). Illustrations of the phenomena can be found in Figure 6. Research has shown refraction effects to be quite substantial. One example study showed that lev- els vary generally less than ±5 dB within about 200 ft from a highway and as much at ±10 dB at 1,000 ft. The highest sound levels were measured under temperature inversion conditions at sunrise (Saurenman et al., 2005).
Shielding can have a strong effect on received sound levels and is another phenomenon that needs to be considered for both highway traffic noise measurements and predictions.
Figure 6. Various atmospheric conditions and their effect on high- way traffic noise.Top left: neutral; top right: upwind (right side) and downwind (left side) conditions; bottom: two temperature profiles - temperature inversion, warmer in air (left) and temperature lapse, warmer at ground (right). Illustration modified from Highway Traf- fic Noise, NHI course 142051.
Highway traffic noise propagation is affected by natural or man-made features or objects that fully or partially block the sound path, leading to reduced sound levels. Features that shield sound include natural terrain, densely wooded areas, large buildings, rows of houses, the top edge of a de- pressed roadway, the edge of a roadway embankment, safety barriers, retaining walls, and noise barriers or berms. There are several paths sound can take when encountering a bar- rier where each one can affect the received sound level: (1) reflected from the barrier back across the highway (reduces noise behind the barrier but can increase noise on the oppo- site side of the highway, particularly when an absorptive sur- face on the barrier is not applied); (2) diffracted over the top of the barrier (reduces noise compared with the direct path, but some of the sound can bend over the top of the barrier, particularly lower frequencies); or (3) transmitted through a barrier (most noise barriers used for noise control provide at least a 30 dB transmission loss, so transmitted sound should not be an issue). Typical highway studies do not consider the scattering of sound; however, scattering can occur from site features such as rough ground surfaces and tree leaves, generally affecting higher frequencies.
Highway Traffic Noise
Metrics and Measurements
There are some generally accepted practices for quantifying highway traffic noise. In the United States, for quantifying highway traffic noise in communities, the A-weighted equiv- alent sound level (LAeqT or LAEQ), the day-night average sound level (Ldn or DNL), the community noise equivalent level (Lden or CNEL), and the percent exceeded sound level (Lx, e.g., L10 for 10%) are commonly applied. For each high- way project, the appropriate metrics for measurements and predictions are determined based on local and state noise policies, where the latter are based on federal regulations.
The equivalent sound level represents an average of the sound energy over a specified period of time. For high- way projects receiving US federal aid, the worst noise hour (LAeq1h) is examined to determine potential adverse impacts on nearby communities. Note that the Federal Transit Ad- ministration (FTA) also applies LAeq1h for institutional land uses, which is relevant in cases of multimodal projects (proj- ects that include multiple modes of transportation; Hanson et al., 2006). Another acceptable metric for federal-aid high- way projects is when the sound level exceeds 10% of the time for the worst noise hour (L10).
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