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Highway Traffic Noise
Figure 4. Car (top) and heavy truck (bottom) one-third octave- band pass-by noise with various pavement types/thicknesses. DGAC, dense-graded asphalt; OGAC, open-graded asphalt; RAC, rubber- ized asphalt. Results are from Caltrans study (Rochat and Read, 2009).
Highway Traffic Noise Propagation
As highway traffic noise propagates away from vehicles to nearby homes or other noise-sensitive receptors, the follow- ing phenomena affect the received sound levels: geometric divergence, ground effects, atmospheric effects, and shield- ing and/or scattering by natural and man-made features.
Although individual vehicles are treated as point sources with spherical divergence, highway traffic noise is treated as a line source with cylindrical divergence (multiple mov- ing point sources along a line behave as a line source). So although the sound from individual vehicles passing by de- creases at a rate of about 6 dB for each doubling of distance, highway traffic noise decreases at a lower rate of about 3 dB for each doubling of distance. Considering only the cylindri- 40 | Acoustics Today | Winter 2016
cal divergence phenomenon, this means that a highway traf- fic noise level of 75 dBA at 50 feet will reduce to 72 dBA at 100 feet and 69 dBA at 200 feet, a typical setback for homes adjacent to a highway. Of course, other phenomena will also influence the received sound level. (Highway traffic noise metrics are discussed in Highway Traffic Noise Metrics and Measurements.)
The ground between vehicles on a highway and noise-sen- sitive land uses can have a measurable and often significant influence on received noise. Figure 5 shows both the direct
Figure 5. Highway traffic noise propagation – direct and ground- reflected sound. Illustration modified from Highway Traffic Noise, National Highway Institute (NHI) course 142051.
and reflected paths as sound propagates away from a vehicle. Sound from both paths can reach the receiver. Grass, loose dirt, and other acoustically soft surfaces can absorb some of the sound as it interacts with the ground, particularly for the shallow angles associated with typical highway/adjacent community geometries. Early highway noise practice ap- plied an additional 1.5 dB reduction per doubling of distance for predictions (beyond cylindrical divergence), but current practice applies greater ground influence with more refined levels based on specific ground types and more sophisti- cated calculations. Although not considered “soft,” ground surfaces such as porous pavement can reduce highway traffic noise at some dominant frequencies. Dense pavements, wa- ter, and other acoustically hard surfaces more effectively re- flect the sound, which then contributes more to the received sound levels compared with soft surfaces. When measuring and predicting highway traffic noise, all ground surfaces be- tween the source and receiver need to be considered, includ- ing the highway traffic lanes and shoulders. In addition, sea- sonal changes might be considered. For example, powdered snow can be very absorptive and water-saturated grass can be very reflective.
Atmospheric parameters that can affect highway traffic noise propagation include air absorption, humidity, and refraction. As with ground effects, the atmospheric effects need to be considered for both noise predictions and mea- surements. Air absorption mostly affects frequencies above 2,000 Hz, with a greater effect over longer distances. Hu- midity affects propagated noise levels but typically to a small