Division: Pure diffraction - Commission Directive (EU) 2015/996 of 19 May 2015 establishing common noise assessment methods according to Directive 2002/49/EC of the European Parliament and of the Council (Text with EEA relevance)

For pure diffraction, with no ground effects, the attenuation is given by:

Δ_dif =	$10 C_{h} \times \lg (3 + \frac{40}{λ} C″δ)$	if $\frac{40}{λ} C″δ \geq - 2$	(2.5.21)
	0	otherwise

where

C_h = 1

(2.5.22)

λ is the wavelength at the nominal centre frequency of the frequency band considered;

δ is the path difference between the diffracted path and the direct path (see next subsection on calculation of the path difference);

C″ is a coefficient used to take into account multiple diffractions:

C″ = 1 for a single diffraction.

For a multiple diffraction, if e is the total distance along the path, O1 to O2 + O2 to O3 + O3 to O4 from the ‘rubber band method’, (see Figures 2.5.d and 2.5.f) and if e exceeds 0,3 m (otherwise C″ = 1), this coefficient is defined by:

(2.5.23)

The values of Δ_dif shall be bound:

if Δ _dif < 0: Δ _dif = 0 dB
if Δ _dif > 25: Δ _dif = 25 dB for a diffraction on a horizontal edge and only on the term Δ_dif which figures in the calculation of A_dif . This upper bound shall not be applied in the Δ_dif terms that intervene in the calculation of Δ _ground , or for a diffraction on a vertical edge (lateral diffraction) in the case of industrial noise mapping.

Division Pure diffraction