Figure 20. Minimum separation distance between two similar vehicles. An XY graph is shown. The distance along the centerline relative to two vehicles' closest point-of-approach to measurement microphone (in feet) is along the X-axis. The direction of travel is from left to right. The relative noise level, in decibels, is on the Y-axis. The background sound level is graphed as a constant 10 dB of which 0.4 dB is due to contamination. The distance between Vehicle 1 and 2 is 93.9 meters. The graph of Vehicle 1 originates at (-500 feet, 0 decibels) and increases exponentially until it peaks at about 18 decibels at 0 feet. The Vehicle 1 graph then begins to decrease exponentially at 50 feet until it reaches 0 decibels at 500 feet. The graph of Vehicle 2 originates at (-800 feet, 0 decibels) and increases exponentially until it peaks at about 18 decibels at -300 feet. The Vehicle 2 graph then begins to decrease exponentially at -250 feet until it reaches 0 decibels at 200 feet. The graphs of the two vehicles intersect at 10 decibels. The graph of the maximum sound level is the sum of Vehicles 1 and 2, plus background, and originates at (-800 feet, 10 decibels) and increases exponentially until it peaks at about 18.5 decibels (.5 decibels of this value is due to contamination) at -300 feet. The graph then begins to decrease exponentially at -225 feet until it reaches 12.5 decibels at -150 feet. The graph, again, increases exponentially until it peaks at about 18.5 decibels at 0 feet. The graph then begins to decrease exponentially at 50 feet until it reaches 10 decibels at 400 feet.

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