*2.3.6 Ground attenuation* Agr

The attenuation Agr is mostly the result of the interference between direct and reflected sound waves. This attenuation is mainly determined by the ground surface near the source and receiver. The calculation method proposed in [5] is only applicable if the terrain is flat, either horizontal or with a constant slope.

In order to calculate the attenuation due to ground absorption, three regions are defined (see **Figure 3**).


The total ground attenuation is obtained for each octave band, by adding the attenuations occurring in the three abovementioned zones. See Eq. (11)

**Figure 3.**

*Regions to calculate the attenuation due to ground absorption (based on [5]).*

$$\mathbf{A\_{gr}} = \mathbf{A\_s} + \mathbf{A\_m} + \mathbf{A\_r} \tag{11}$$

Where:

Agr: Total sound absorption due to ground effects (dB)

As: Sound absorption due to ground effects at the source region (dB)

Am Sound absorption due to ground effects at the middle region (dB)

Ar: Sound absorption due to ground effects at the receiver region (dB)

ISO Standard 9613-2 [5] explains in detail how to calculate the values of As, Am, and Ar. In the expressions for calculating the attenuation, the acoustic properties of each of these zones are taken into account through the so-called "G factor." When the sound is expected to propagate over hard ground: G = 0; for porous or soft ground: G = 1; and for mixed soil along the sound path, G should take a value between 0 and 1.
