Division Acoustic characterisation of ground
United Kingdom
The acoustic absorption properties of the ground are mainly linked to its porosity. Compact ground is generally reflective and porous ground is absorbent.
For operational calculation requirements, the acoustic absorption of a ground is represented by a dimensionless coefficient G, between 0 and 1. G is independent of the frequency. Table 2.5.a gives the G values for the ground outdoors. In general, the average of the coefficient G over a path takes values between 0 and 1.
Table 2.5.a
G values for different types of ground
| Description | Type | (kPa · s/m2) | G value |
|---|---|---|---|
| Very soft (snow or moss-like) | A | 12,5 | 1 |
| Soft forest floor (short, dense heather-like or thick moss) | B | 31,5 | 1 |
| Uncompacted, loose ground (turf, grass, loose soil) | C | 80 | 1 |
| Normal uncompacted ground (forest floors, pasture field) | D | 200 | 1 |
| Compacted field and gravel (compacted lawns, park area) | E | 500 | 0,7 |
| Compacted dense ground (gravel road, car park) | F | 2 000 | 0,3 |
| Hard surfaces (most normal asphalt, concrete) | G | 20 000 | 0 |
| Very hard and dense surfaces (dense asphalt, concrete, water) | H | 200 000 | 0 |
Gpath is defined as the fraction of absorbent ground present over the entire path covered.
When the source and receiver are close-by so that dp ≤ 30(zs + zr ), the distinction between the type of ground located near the source and the type of ground located near the receiver is negligible. To take this comment into account, the ground factor Gpath is therefore ultimately corrected as follows:
| G′path = | if dp ≤ 30(zs + zr ) | (2.5.14) | |
| Gpath | otherwise |
where Gs is the ground factor of the source area. Gs = 0 for road platformsf1, slab tracks. Gs = 1 for rail tracks on ballast. There is no general answer in the case of industrial sources and plants.
G may be linked to the flow resistivity.
Figure 2.5.b
Determination of the ground coefficient Gpath over a propagation path
The following two subsections on calculations in homogeneous and favourable conditions introduce the generic Ḡw and Ḡm notations for the absorption of the ground. Table 2.5.b gives the correspondence between these notations and the Gpath and G′path variables.
Table 2.5.b
Correspondence between Ḡw and Ḡm and (Gpath , G′path )
| Homogeneous conditions | Favourable conditions | |||||
|---|---|---|---|---|---|---|
| Aground | Δground(S,O) | Δground(O,R) | Ag round | Δground(S,O) | Δground(O,R) | |
| Ḡw | G′path | Gpath | ||||
| Ḡm | G′path | Gpath | G′path | Gpath | ||
Annotations
Footnotes
- f1
-
The absorption of porous road pavements is taken into account in the emission model.