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Effect of Distance and Air Absorption

In the following, the effects of geometrical divergence and of air absorption are examined.

  • Open the dialog Geometry of the point source once more. Enter a height of 1000 m for the point source.

Note

Due to this height, there is influence of the ground attenuation and the ground reflection on the calculation result.

  • Choose the object "receiver" from the toolbox. Enter - in the same way like for the point source - the coordinates (x, y) = (200,100).
  • Double-click on the border of the receiver point to open the dialog.
  • Set the receiver height on the Geometry dialog to 1000 m.
  • Activate the option "Generate Rays (as Aux. Polygons)" at the lower end of the dialog.

Point source and receiver point being entered

  • Start the calculation by clicking on the pocket calculator symbol on the symbol bar ().

  • Then, open the dialog of the receiver point to check for the calculated sound pressure level.

Receiver: resulting level LpA = 48,8 dB(A)

Since the ground attenuation (Agr) and the reflection (Domega) can be neglected at a height of 1000 m, the calculated sound pressure level is just due to divergence and air absorption.

\(L_{AT}=L_{wA}-A_{div}-A_{atm}\)

Thus:

\(L_{AT}=100dB(A)-[20\lg (100m/1m)+11]dB-0.002dB/m*100m\)

\(L_{AT}=100dB(A) - 51 dB - 0.2 dB = 48.8 dB(A)\)