PLANNING FOUNDATIONS
2.5 The effect of noise protection constructions

The appropriate geometrical arrangement of obstacles (noise barriers, walls, buildings etc.) can effectively reduce the influence of a sound source on a place of immission. Similar to the shadow of light, an acoustic shadow is formed behind the obstacle but is reduced by sound diffraction at the edges.

Figure 2/15 gives a diagram of the shielding effect of a sound source.

What determines the noise level reduction is the path length difference z. The path length difference is the diversion of the acoustic beam around an obstacle (z = A + B - b - a). The parameter z is substantially determined by the obstacle's effective height h_eff, but also by the distance of the barrier to the sound source.

The path length difference is calculated according to the following formula:

The noise level reduction through a barrier can be roughly estimated in accordance with the following equation:

∆L_z = 10 lg(3 + 0.12 · f · z)

The frequency f in Hz is assumed at 500 Hz for industrial noises for example.

Example: a = 15 m   b = 35 m   heff = 2 m     z = 0,19   ∆Lz = 10 lg (3 + 0,12 · 500 · 0,19)   ∆Lz » 11 dB

The diagram in figure 2/16 also allows for an estimation if h_eff and the distance e between sound source and place of immission (flat) are known. Thereby, h_max means the height of a place of immission above the street which can still be efficiently protected (depending on the width of the street and the distance of the place of immission to the street). More detailed calculation directives are given in VDI 2720-1, RLS-90 and Schall 03.

Figure 2/16 shows that the wall does not completely hold off the sound but bends it at the top edge. As this is also true for the sides of the wall, a sufficient length is important here. This schematic representation also applies to noise protection barriers with the wall's top edge being replaced by the top of the barrier here.

On the basis of the above-mentioned principles, the following indications and recommendations for the construction and structuring of barriers can be set up:

• The effective barrier height, i.e. the camber, is decisive for the level reduction as this factor is squared and thus has a bigger influence on the parameter z.
• Barriers should be placed as close to the sound source as possible; height and length of the barrier can then be reduced without losing efficiency.
• The more distance there is between place of immission and barrier, the more the noise level reduction decreases. The efficiency of a barrier in a distance of more than 400 m is very low.
• Barriers should guarantee an average noise level reduction of at least 5 dB.
• Barriers in immediate proximity of the place of immission are indeed effective but they are often perceived as disturbing (obscuring visibility, obstructing sunlight).
• Barriers must not only be high enough but also long enough as sound is also bent at the sides.
• Capital expenditure, space requirement, maintenance costs and aesthetic requirements are to be optimized.
• The distance at the top of the barrier from the source is bigger than in the case of a wall due to the barrier's width at its bottom. Therefore, noise barriers must generally be higher than noise protection walls.
• Noise protection barriers require more space than walls. Their width at the bottom is generally three or four times their height, which increases land acquisition costs as well as nature and landscape interferences.
• In order to avoid sound reflections towards residential areas requiring protection, it is often necessary to use sound-absorbing surfaces.