Meterological Excess Attenuation

Am, the excess attenuation due to meteorology is obtained by firstly calculating the vertical sonic gradient 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.

The calculation of the vertical sonic gradient may be found in many sources. It is a function of both the vertical temperature gradient and the wind gradient.

In Reynolds, D.D., “Engineering Principles of Acoustics – Noise and Vibration Control”,Allyn and Bacon Inc, Boston, 1981 the vertical wind velocity profile is given by,

This equation is used for terrains that have tall vegetation and only has meaning if z is greater than or equal to z0+d. It has found to be accurate in the zone close to the ground.

The temperature profile varies according to the time of day and can be very complex. However, for simplicity, it is assumed that the vertical temperature gradient is constant as in [1] Tonin, R, “Estimating Noise Levels from Petrochemical Plants, Mines and Industrial Complexes”, Acoustics Australia, 13(2):59-67, 1985.

If the total vertical sonic speed gradient is zero then the meteorological excess attenuation will also be zero. A sonic gradient of +0.15 represents saturation and represents the most negative value of the range. A sonic gradient of -0.15 also represents saturation and represents the most positive value of the range. For all other sonic gradients the meteorological effects are linearly interpolated. For distances more than 1000 m the 1000 m value applies and for distances less than 100 m the 100 m values apply.

Variability in Sound Level Predictions due to Meteorological Influences
Octave Band Center Frequency (Hz)	Attenuation in dB at Various Distances From Source (m)
	100	200	500	1000
63	+1 to -1	+4 to -2	+7 to -2	+8 to -2
125	+1 to -1	+4 to -2	+6 to -4	+7 to -4
250	+3 to -1	+5 to -3	+6 to -5	+7 t o -6
500	+3 to -1	+6 to -3	+7 to -5	+9 to -7
1000	+7 to -1	+11 to -3	+12 to -5	+12 to -5
2000	+2 to -3	+5 to -4	+7 to -5	+7 to -5
4000	+2 to -1	+6 to -4	+8 to -6	+9 to -7
80000	+2 to -1	+6 to -4	+8 to -6	+9 to -7

The variability in sound level predictions 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.