Natural Science Forum / Physics / Acoustics / August 2005

An earthen berm or bank could be an effective barrier if it is high enough
and if you are dealing with distances in which a barrier can be effective.

Foliage effects are not nearly as strong as once thought.  Much of the
effect once attributed to foliage was found to be actually an effect of soft
ground usually present under foliage.  Foliage of some sort, possibly just
grass, is usually needed to assure soft ground.  Thus, you might say there
is a relationship between the ground effect and foliage, but it is the
ground effect that provides most of the benefit.  Like the barrier effect,
this is most effective over distances before the effect of atmopheric
bending becomes dominant.

The top of the bank has to break the direct path of sound, which is not a
straight line but curved up or down due to temperature and wind gradients.
The bank surface should be soft (grass).
Foliage in trees on top of a bank or screen may destroy the screen effect
due to sound reflections and scattering.

Asbjørn

Richard, previous postings are entirely correct.  If you're really keen
you could use 'The Calculation of Road Traffic Noise' issued in the UK
by the Department of Transport and the Welsh Office, 1988 (first
published 1975) or 'The Calculation of Railway Noise 1995' to calculate
the actual noise reduction due to a solid barrier (which includes and
earth bund) which breaks the line of sight between the source and the
receiver.  As a rough rule of thumb, if the line of sight is just
broken then there will be a noise reduction of about 5 dB(A), which
should prove just noticeable.  If the line of sight is obscured then
the reduction should be about 10 dB(A), which subjectively would appear
to be a reduction of about half at the receiver point.  Substantial
noise barriers (those along the M25, for example) can reduce noise by
up to 20 dB(A) or so.

Neither of the above calculation methods allows for any on-bund
additional absorption due to vegetation, but research clearly indicates
that a vegetated earth bund will offer slightly more noise reduction
than an equivalent noise barrier (solid wall, fence etc.) offering the
same obstruction (height etc.).

Foliage (bushes, trees etc.) between source and reciever over flat
ground offers minimal additional noise reduction compared to, say,
grass but would be better than transmission over hard ground (such as
water or concrete).  There is a soft-ground absorption correction
factor in 'Calculation of Road Traffic Noise' or British Standard 5288.
Trees offer minimal additional noise shielding compared to grass or
vegetation, unless very substantially planted (60m deep belt, thick
trunks, no line-of-sight through) and the notion that trees, bushes
etc. planted between neighbours, in front gardens etc. will reduce
noise is incorrect.

I could go on, but this is getting boring.....and I must get out
more....

Nic