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Pages 3-11

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From page 3...
... 3 Individuals who live near highways probably notice that vehicular traffic sounds change hourly and daily. Some sound changes are attributable to the number of cars and trucks on the road, but meteorology also affects how sound travels or propagates from the roadway.
From page 4...
... 4 How Weather Affects the Noise You Hear from Highways Another problem often encountered is the inability to accurately model traffic noise for individuals further removed from the immediate vicinity of the highway, but who are more affected by meteorological effects. Measurements have shown large fluctuations (and often increased sound levels)
From page 5...
... Background 5 Using a typical frequency spectrum of a heavy truck at 65 mph, the calculated A-weighted sound pressure level at 1 km away (assuming no ground attenuation and no meteorological effects) are calculated as a function of relative humidity and temperature in Figure 1 (right)
From page 6...
... 6 How Weather Affects the Noise You Hear from Highways Figure 2 illustrates the effects of refraction. The figure shows a sound source near the ground in an atmosphere with wind directed to the right.
From page 7...
... Background 7 Source: RSG for NCHRP Project 25-52. Figure 4.
From page 8...
... 8 How Weather Affects the Noise You Hear from Highways The performance of noise barriers is also influenced by atmospheric effects. The performance of a barrier in some situations can be reduced by downward refraction, since sound is directed downward as it travels over a barrier along downward-curved sound rays and into the shadow zone.
From page 9...
... Background 9 These methods, described in more detail in Appendices A and B, are computationally intensive and not appropriate for engineering models. That is, they require a great deal of data that is usually not available without great cost; these procedures also take too long to run to make them useful to the general practitioner in a regulatory setting.
From page 10...
... 10 How Weather Affects the Noise You Hear from Highways While this is the case, several studies in the United States have evaluated the effect of meteorology on highway traffic noise. Roger Wayson collected a large database of measured meteorological and sound level variables near a highway.
From page 11...
... Background 11 Pitfalls to Avoid Based on this overview from the previous section, it is apparent that modeling has multiple pitfalls that need to be avoided, whether due to extremes or just simple oversights. The extremes occur where a model may be based on sound theoretical analysis but is too complex and computer intensive to be adapted for normal highway project use.

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