computer, especially if a large number of positions along a road have to be considered. Several calculation methods are available. A guide which gives detailed instructions for both a manual and a computer method of noise prediction has been produced in Alberta, Canada. The Leq nomograph shown in Figure 660-1 can be used to illustrate a simple example as follows
1 A straight line is drawn from the starting point through 60 mph on the auto speed scale to the turn line
2. A second straight line is drawn from this turning point to the point on the volume scale indicating 5000 vehicles per hour. The intersection of this straight line with the pivot line is boxed,
3. A third straight line is drawn from this box to the 500 ft point on the distance scale.
4. The intersection of this line with the Un scale gives the predicted L«i at the observer: 65 dB(A).
This method can be repeated for both medium and heavy trucks, and the resultant of all three values is obtained by using the dB additions method shown in Table 660-3.
A completed calculation of noise prediction involves additional calculations which take into account road and noise propagation characteristics. A comprehensive traffic noise study typically involves calculating the Lct| for both the day and night periods and then combining them to obtain the Ldn.
The Leq nomograph used for traffic noise can also be used for train noise prediction (Figure 660-1). Adjustments to the basic calculated level have to be made for such factors as the existence of welded tracks or whether or not the railway crosses a steel structure (I.e., an overpass).
Aircraft noise predictions are usually represented by a set of noise contours for areas surrounding the airport. Several methods for rating aircraft noise exposure have been developed, including:
NEF noise exposure forecast
NNI noise and number index
CNR composite noise rating
Ldn day-night sound level
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