When a noise doesn't yet exist, or it is planned to alter an existing noise
climate, the ability to calculate the future [or even existing] noise level can
be vital. Noise calculations are complex and have many traps for the unwary;
this article considers calculations only briefly; further UK reading is contained
- The Calculation of Road Traffic Noise" a DoE and
available from HMSO.
"Noise and vibration control on construction and open
- "The Control of Noise from Surface Minerals Workings" a DoE
report also from HMSO.
- CONCAWE Report 4/81 "The Propagation of Noise from Petroleum and
Petrochemical Complexes to Neighbouring Communities".
Information on the related
topic of noise maps and noise mapping can be found elsewhere
on the DETR website.
There are many factors which affect the level of noise which reaches any
given point, but the most important, over which there is generally some control
For most sources a doubling of the distance results in a 6 dBA fall in level;
conversely halving the distance, produces a 6 dBA rise. For example if you
are 5m from a noise source, then moving another 5m away, should make a large
difference to the measured noise level; conversely if you are 500m away from the
noise source, than a 5m change in distance will make no difference. The main
exceptions to this rule are either when you are proportionally close to a large
noise source and the drop off in noise will be significantly less than 6 dBA per
doubling of distance, or for a line source (e.g road traffic) where the drop-off
is only 3 dBA per doubling of distance.
For a barrier to be effective it has to cut the line of sight between the
noise source and the reception point; if it just cuts the line of sight a
5 dBA reduction might be expected, if it significantly cuts the
line-of-sight then anything from 10 dBA, up to 20 dBA reduction can be expected.
You should also bear in mind that the noise has got to be "forced over the
top", i.e. lightweight barriers, or ones with holes in, permit noise to
pass through the barrier and are unlikely to be effective; for example domestic
larch or waney lap fencing simply isn't heavy enough, and has too many gaps and
cracks to be an efficient noise barrier; by contrast specialist acoustic fencing
[consult Directory for suppliers] and brick walls are heavy enough and shouldn't have
Hard reflecting surfaces close to either source or reception point can
increase noise levels by up to 3 dBA [each]. Essentially there are equal two
noises which add together [see below]; they are the noise which travelled
directly and the noise reflected from the hard surface. Sometimes noise barriers
can reflect sound off in unexpected and unwanted directions. It is possible to
design absorptive barriers which do not have this reflection effect, the Directory
can be consulted for specialist manufacturers.
"Soft Ground Attenuation"
When sound passes close to absorbing ground cover such as grassland, gardens,
fields etc., this "soft ground" absorbs extra sound as it passes.
Examples of "hard ground" where little no absorption occurs are
asphalt, water and paving. The absence of this effect is particularly noticeable
in balloons (I believe!) where sounds from far below sound loud and clear
because of the absence of soft ground (and other) attenuation.