Division Favourable conditions
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Figure 2.5.e
Calculation of the path difference in favourable conditions (single diffraction)
In favourable conditions, it is considered that the three curved sound rays SO, OR, and SR have an identical radius of curvature Γ defined by:
| Γ = max(1 000,8d) | (2.5.24) |
The length of a sound ray curve MN is noted M̂N in favourable conditions. This length is equal to:
 | (2.5.25) |
In principle, three scenarios should be considered in the calculation of the path difference in favourable conditions δF (see Figure 2.5.e). In practice, two equations are sufficient:
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if the straight sound ray SR is masked by the obstacle (1st and 2nd case in Figure 2.5.e):
(2.5.26) -
if the straight sound ray SR is not masked by the obstacle (3rd case in Figure 2.5.e):
(2.5.27)
where A is the intersection of the straight sound ray SR and the extension of the diffracting obstacle.
For the multiple diffractions in favourable conditions:
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determine the convex hull defined by the various potential diffraction edges;
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eliminate the diffraction edges which are not on the boundary of the convex hull;
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calculate δF based on the lengths of the curved sound ray, by breaking down the diffracted path into as many curved segments as necessary (see Figure 2.5.f)
(2.5.28)
Figure 2.5.f
Example of calculation of the path difference in favourable conditions, in the case of multiple diffractions
In the scenario presented in Figure 2.5.f, the path difference is:
 | (2.5.29) |