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How many reflections should I use?

Sound level prediction is an important issue for noise assessment.  When calculating the sound level with Standards such as ISO 9613-2 reflections from vertical obstacles often dominate the accuracy for shielded locations. These locations may be sensitive residential areas hidden behind a cluster of industrial buildings or behind one or more noise barriers.  This is the summary of a paper presented by Weigang Wei at the Institute of Acoustics Conference in Harrogate, 2015.

Some commercial software such as CadnaA and Soundplan offer options for users to control the number of reflections used in the calculation. However, increasing the number of reflections requires more calculation time and might not lead to more accurate predictions.  In this paper, the accuracy of applying a different number of reflections is compared by using theoretical calculations and simulations of the ray/ beam tracing method in ISO 9613-2.   The variables include different heights of obstacles and varied topologies of the obstacles or buildings. This study recommends the number of reflections for different building topologies if the ISO 9613-2 propagation model is used for sound level prediction.

Topologies considered

Three typical topologies are considered in this study, which covers most of the acoustical geometrical conditions in urban areas.  These include:

  • Attached or terraced houses
  • Dense detached houses
  • Detached houses with an open area in the rear
Three different scenarios for topology when calculating the propagation of sound

Model details

Two arrangements of sound sources are considered in this study. The first one is a single point source which may represent one unit of mechanical plant. The other one is a “line source”, which is represented in the model by of series of point sources which may represent traffic on a road.

Handling small objects

Small objects or surfaces are often neglected when calculating reflections according to the ISO 9613-2 method.  The equation that determines whether or not reflections from an object are included is shown below.  When an object’s length is less than 2 m, there is a high possibility that the calculation may neglect the existence of this object, depending on the surrounding geometry and other variables.  The graph illustrates the possibility of a reflection being included in the calculation as a function of frequency and dimension lmin.  With this understanding an experienced operator will be able to appropriately model the surrounding geometry and topography in any given situation. 

Equation used in ISO 9613 to determine if reflections from objects are included.

Conclusion

The paper describes the minimum number of reflections that are suggested in the table below.

Suggested number of reflections, depending on the geometry modelled

Download the full paper here:

Acoustics2015 – How many reflections should I use

Posted in: Environmental noise modelling research

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