Floating Barrier
The floating barrier is a special object which is not described in the existing standards or guidelines for calculating the sound propagation outdoors. Originally, it has been designed to be used for modeling barriers on bridges (see Bridge).
Due to further development of CadnaA this application is obsolete meanwhile because barriers on bridges or viaducts can be modeled more comfortably by using the option „Self-Screening“ of roads (see Common Input Data) and railways (see Common Input Data).
Nevertheless, the floating barrier can be used to model - for example - elevated components in a plant (e.g. elevated horizontal or vertical tanks or containers).
Option "Floating Barrier"
If this option is enabled, then the lower edge of the barrier is above the ground level. Thus, this barrier is no longer attached to the ground.
Z-Extent (m)
In this input box - in addition to the geometry defined on the Geometry dialog - the vertical extent from the top edge to the bottom edge of the floating barrier is entered.
Example
With a barrier height (upper edge) of 10 m relative and as a Z-extent of 3 m the height of the barrier’s lower edge is 7 m above ground.
Properties of the floating barrier
A floating barrier with the lower edge lifted from the ground has the following properties:
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The screening effect is calculated - depending on the selected standard/guideline - for each ray hitting the barrier surface, considering all three propagation paths (diffraction across the top edge and laterally around the barrier).
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Rays passing underneath the barrier’s lower edge are not attenuated (i.e. free propagation).
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A diffraction effect around the bottom edge is not taken into account.
Note
The diffraction algorithms found in the present standards and guidelines normally just hold for a barrier’s upper edge standing vertically on the ground. The diffraction around the lower edge cannot be calculated using these algorithms, as the more complex interaction with the ground is not included. Since a diffraction model is missing in the standards, CadnaA does neither consider a diffraction effect.
- Reflections of rays hitting the barrier’s surface are calculated according to the image source method.
Barrier on a Bridge
The procedure described below when entering a barrier on road or railway bridges is superfluous by using the option „Self-Screening“ provided for the object types „Road“ and „Railway“ (see Common Input Data and Common Input Data, for both in Self-Screening).
When a barrier is located on a bridge, the height of the barrier section on the bridge must be entered either by entering
- the height of the top edge above the base plate of the bridge, and the „Building Height“ button in the Geometry dialog must be activated - that means 3 m in our example - and the same value must be entered for the z dimension after „Floating Barrier“ has been activated, or
- the height of the top edge as an absolute value (13 m in the example. The z dimension remains the same (3 m).
Note
In both cases, further polygon points must be inserted for the barrier where it intersects contour lines.
The barrier is calculated correctly in both cases.
Inserting a barrier on a bridge
Example
Path: Examples\Obstacles\Bridge.cna
- In the two dialogs below enter the height of a reflecting barrier as 3 m above bridge level.
3-m-high barrier on a bridge in the 3-D front view
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The vertical grid shows the level distribution due to the reflection at the barrier (in the upper right half sphere) and the screening effect caused by the barrier on the left side of the bridge.
Sound level distribution on a vertical grid for a road on a bridge with a floating barrier
see also: