There are a number of quite computationally intensive methods for calculating this reduction, however, a simple method that suits our purposes is as follows;

R = 10 log(3 + ((40fd) /c)) 

An alternative method is to derive the term (u) for which charts exist relating this to dB reduction.

u = (1.414h / sqrt(l)) * sqrt((a + b) / (a * b))  

Note:  This prediction is based on the assumption that any transmission through the wall or screen itself is negligible.

Barrier Placement

• It is apparent that a given barrier will be most effective if placed near the source.
• The second best position would be near the receiver.
• The least effect will occur if the barrier is half-way between them.

Vegetation as a Barrier

It appears that there is a widely held belief that vegetation is an effective controller of sound. However, research by Fricke (1984) and others shows that vegetation is only effective at higher frequencies (above 2000Hz). The impedance of the ground is the dominant factor at lower frequencies (125-500Hz) with vegetation having little effect on sound propagation at mid-frequencies (500-2000Hz). It is suggested by Aylor (1972) that scattering, rather than absorption, is the more important phenomenon at mid-frequencies. At higher frequencies, however, absorption takes over as the dominant phenomenon.

The literature generally suggests that the principal effect of plantings is psychological. By removing the noise source from view, plantings can reduce human annoyance to noise. The fact that people cnnot see a highway can reduce their awareness of it, even though the noise remains.

Large areas of vegetation can be effective, but only in belts of greater than 50m.