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Ground Absorption

The calculation of ground absorption is determined by the standard or guideline chosen for each noise type. With modeling of industrial sources according to ISO 9613-2 (1996) (or ISO 9613-2 (2024)) two ground attenuation models have to be distinguished:

  1. spectral method according to section 7.3.1 of ISO 9613-2 with the ground factor G as a parameter, and

  2. method for A-weighted levels according to section 7.3.2 of ISO 9613-2 not using the ground factor G.

On the toolbox the object „Ground Absorption“ (symbol: ) enables one to define areas with a specific ground factor G when applying the spectral method of ISO 9613-2. Consequently, these areas are not relevant for the method for A-weighted levels of ISO 9613-2.

Default Ground Absorption

Open on dialog Configuration (Calculation menu) the tab „Ground Absorption“. The „default ground absorption“ applies to all areas within the limits not being defined by ground absorption areas. The „default ground absorption“ (ground factor) is defined to G=1 by default (i.e. "porous ground“).

Tab Ground Absorption: specifying default ground absorption

Tab Industry

With the industrial guideline ISO 9613(1996) (as well as ISO 9613 (2024)) several methods can be selected from the dialog Configuration, tab „Industry“.

Tab Industry: options for ground absorption

The following four options are available:

Selection Procedure
none No ground attenuation Agr is calculated. The ground reflection DΩ , however, is considered using equation (11).
not spectral The ground attenuation Agr is calculated for spectral and non-spectral sources is calculated according to the method described in section 7.3.2.
spectral, spectral sources only For spectral sources the method according to section 7.3.1, in all other cases the method according to section 7.3.2 is applied.
spectral, all sources For spectral and for non-spectral sources the method according to section 7.3.1 is used. For non-spectral sources the ground attenuation is calculated based on the specified octave band center frequency.
- 3.0 dB A fixed value for Agr of -3 dB is used with A-weighted or for all octaves resp. third octaves with spectral calculations (special procedure for wind turbine noise assessments in Germany).

By default, the option „spectral, all sources“ is selected. Keep this setting for all further calculations.

Specifying areas of ground absorption

  • Select the object „Ground Absorption“ from the toolbox and enter an approximately circular area in the middle of the screen (scale 1:1000).
  • In order to smooth the contour select the command Spline from the context menu of the ground absorption area.
  • Enter a point source with an A-weighted sound power level of 100 dB (A) to the left of this area.
  • Define a calculation area surrounding the source, the ground absorption area and the area on the right of the ground absorption area (as displayed on the following figure).

Calculation area with point source and ground absorption area

Reflecting ground absorption area

  • Double-click on the border of the ground absorption area. By default, "hard ground" is predefined (ground factor G=0). Keep this value and close the dialog.
  • Open on dialog Configuration (Calculation menu) the tab „Ground Absorption“.

Keep the „default ground absorption“ (ground factor) of G=1 (i.e. "porous ground“). Thus, a reflective area inside the absorbing limits is assumed.

  • Adjust the grid spacing to 1x1 m (menu Grid|Specification) and select „Lines of Equal Sound Level“ and activate the option „Level Range“ (menu Grid|Appearance).
  • Run the grid calculation via the Grid menu (command Calc Grid).

Grid with reflecting ground absorption area

The resulting grid shows the lines of equal sound level bent to the outside when looking from the source’s location. Because the area being reflective the sound energy is reflected by the ground leading to a level increase compared with the absorbing case.

Absorbing ground absorption area

  • Open again the tab „Ground Absorption“ (menu Calculation| Configuration) and change the default ground absorption to G=0.
  • Double-click on the border of the ground absorption area and specify a ground factor of G = 1.
  • Start the grid calculation again.

In this case, the lines of equal sound level are bent towards the source’s location due to the sound energy absorbed with distance - compared to the reflecting situation outside that ground absorption area.

Grid with absorbing ground absorption area

Further options on tab Ground Absorption

For information on the further options on tab „Ground Absorption“ see the CadnaA-reference manual, chapter Ground Absorption Tab.