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CNOSSOS Reflection    1/1,1/3

Strutt|Environmental Noise|CNOSSOS Reflection inserts the reflection strength (in dB relative to the direct sound) for a finite-sized reflector into the active row of the worksheet calculated using the CNOSSOS EU noise propagation model.

The CNOSSOS reflection loss is termed "retrodiffraction" in the CNOSSOS paper, and accounts for the diffraction of some of the sound energy when a reflection occurs close to the edge of a finite-sized reflector - i.e. some of the energy is scattered by diffraction, which reduces the strength of the reflected sound.

The CNOSSOS model predicts the reflection strength Delta L_w of the image source. In the Strutt implementation of CNOSSOS, the distance loss for the reflected path (calculated using the Ratio of Distances calculation for either point or line source propagation) is by default added to Delta L_w to obtain the overall reflection strength relative to the direct sound. This simplifies the calculation by not requiring the user to conduct a completely new calculation using the image source as a new source.

Any significant absorption loss (i.e. Delta L_alpha = 10 log_10(1-alpha)) from the surface should be added manually by the user.

In theory, multiple reflections can be accommodated by successive application of the CNOSSOS reflection calculation to obtain the cumulative retrodiffraction loss.
The distance loss may be turned off, if needed, to simplify calculation of multiple reflections by 'stacking' successive CNOSSOS Reflection calculations.
However, for situations with multiple reflections (including semi-reverberant enclosures/courtyards) any image-source based method should be used with caution. Consider using the Semi-Reverberant Courtyard calculation as an alternate.

The reflection loss is calculated using the path difference delta between the direct sound and the reflected path:
delta = d_(SR)-d_(SO)-d_(OR) - i.e. using a negative value of path difference.
d_(SR) is the source-receiver distance
d_(SO) is the distance from the source to the top of the reflector
d_(OR) is the distance from the top of the reflector to the receiver.

In effect, the reflection calc is treated as a 'flipped' barrier calculation where source and receiver are both on the same side of the barrier:

The 'true' reflection geometry (blue line) is geometrically equivalent to the 'barrier' geometry (orange line) in calculating the path difference for reflection (i.e. orange line minus yellow line).

Because of the negative sign of the path difference delta in the reflection calc, the closer the specular reflection point is to the top of the barrier, the stronger the diffraction loss.

The reflection loss due to diffraction is calculated using the same formula as 'pure' barrier diffraction in CNOSSOS Barrier Attenuation only using the negative path difference, as explained above.

Delta_("diff") = {(10 C_H log_10 (3+40/lambda delta) " if " 40/lambda delta >= -2),(0 " otherwise") :}
where:
lambda is the wavelength
delta is the path difference for reflection)
C_H is a coherence coefficient that reduces the strength of the reflection effect at low frequencies where the reflector size is small relative to wavelength.
C_H="Min"((f h"))/250,1), where
f is the frequency
h is the reflector height

References:

• Stylianos Kephalopoulos, Marco Paviotti, Fabienne Anfosso-Lédée (2012) Common Noise Assessment Methods in Europe (CNOSSOS-EU) EUR 25379 EN. Luxembourg: Publications Office of the European Union, 2012, 180 pp.

Comments or suggestions to
strutt@arup.com