Skip to content

Effect of Ground Absorption

In the standard ISO 9613-2 (1996) two different models for assessing the ground effect (ground attenuation) are described:

  • the spectral model (ISO 9613-2, section 7.3.1), and
  • the model for A-weighted sound pressure levels (section 7.3.2).

When calculating only the A-weighted sound pressure level the so-called „alternative method“ according to ISO 9613-2 (1996) can be applied under the conditions that

  • the sound propagation occurs above porous or mostly porous ground,
  • the sound is not a pure tone.

In this case, the ground attenuation Agr is calculated from:

\(A_{gr}=[4.8-\frac{2h_m}{d}(17+\frac{300}{d})]dB\geq 0dB\)

In order to calculate the ground attenuation for A-weighted levels according to this formula, the setting on tab "industry" (on the dialog Calculation|Configuration, „Ground Attenuation“) has to be either "not spectral" or "spectral, spectral sources only".

Dialog Configuration, tab „Industry“

Note

For more details on the further settings for ground absorption see ISO 9613 (1996) in the CadnaA-reference manual.

Note

With selection "none" no ground attenuation is calculated (Agr = 0 dB), however, the directivity correction according to formula (11), ISO 9613-2 (1996) is considered.

Note

With setting „spectral, all sources“ the frequency dependent model is applied („general method“). For sources the emission of which is specified by the A-weighted sound power level the entered frequency is considered to calculate the ground attenuation (default value: 500 Hz).

  • On the example, reduce the height for the point source to 0.5 m above ground, and for the receiver to 1.5 m.
  • Restart the calculation.

The protocol shows a ground attenuation of Agr = 4.4 dB:

To consider the apparent increase of the sound power level of the source due to ground reflections near the source, the directivity index DΩ is added - resulting from distance and heights of source and receiver (here in column K0 showing +3 dB).

Note

With spectral ground attenuation selected the ground reflection is already taken into account in the calculation of Aground. In this case, the column „K0“ shows zero or - if any - the „K0 w/o Ground“ as entered on the source‘s dialog.

In this example, a sound pressure level of 47.4 dB at the receiver’s location results.