Ab, the excess attenuation due to barriers is obtained by firstly calculating the curved noise path due to wind and temperature effects. It is calculated by reference to the method outlined in “A Method to To Incorporate Meteorological Effects into A Road Traffic Model” by MA Simpson, Proceedings of Acoustics 2004.
If a barrier exists the effective location of the source and receiver is modified according to the method outlined in Tonin, R, “Estimating Noise Levels from Petrochemical Plants, Mines and Industrial Complexes”, Acoustics Australia, 13(2):59-67, 1985.
Method to modify source and receiver location
Barrier attenuation is calculated by the Maekawa formulation.
PEN Implementation
The noise barrier calculation incorporates a sophisticated method to calculate the total sonic gradient. This method incorporates the vertical wind profile for various types of surfaces. The sonic gradient also changes with height above the ground. As a consequence, depending on the type of surface, the sonic gradient will vary over the DTM and vary with height of the noise ray. These issues have been addressed in the PEN model and a most likely curved path is calculated by iteration. Up to 250 possible iterations of the curved path are calculated for each sound ray.
Once the most likely curved path has been calculated the effective source location and effective receiver locations are determined.
The likely barrier attenuations are calculated for four possible paths, namely:
| • | source, top of barrier, receiver; |
| • | source, reflection from ground (source side), top of barrier, receiver; |
| • | source, top of barrier,reflection from ground (receiver side), receiver, and; |
| • | source, reflection from ground (source side), top of barrier, reflection from ground (receiver side), receiver. |
The are combined to obtain an effective barrier attenuation.
Thus in the situation where the source and receiver are well of the ground and the barrier just intercepts line-of-sight then the barrier effect will be 5 dB(A). However, if the source and receiver are close to the ground and the noise barrier just intercepts line of sight (a pebble) the barrier effect will tend to zero.
If there are noise barriers the ground effect is not included in the calculation as this method involves several ground reflections.