NoiseNet Logo

NoiseNet Text

site search
members area
home directory ProInfo noise vibration contact

Noise > Terms > Calculating Levels 

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 in

  • The Calculation of Road Traffic Noise" a DoE and D.Tp. report available from HMSO.
  • BS.5228 "Noise and vibration control on construction and open sites"
  • "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 are:-

Distance
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.

Barriers
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 any holes.

Reflecting Surfaces
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.

Adding Noise Levels Together
Finally when adding or subtracting noise levels, the following table gives the appropriate corrections, to the nearest whole decibel.

Difference between the two levels [dBA] Addition to higher level [dBA]
0 3
1 3
2 2
3 2
4 1
5 1
6 1
7 1
8 1
9 1
10 and over 0
 

As an example if a noise source produces a level of say 53 at a particular location, and another source is to be installed which will produce say 59 at the same monitoring location, then the difference between the two levels is 6 dBA (59 - 53); therefore, from the table, the total level is calculated by addition 1 dB to the higher level, i.e. 60 dBA.

Thus, perversely   53 dBA + 59 dBA = 60 dBA

Note  53 dBA + 59 dBA does not equal 112 dBA!!

Again the Directory can be queried for suppliers of prediction software and suitable consultants.

<<< Back to Introduction

home directory ProInfo noise vibration contact

©NoiseNet.org Ltd 2000 - 2008 Terms & Conditions of Use

 

 

 

 

 

 

 

 

 

 

Visit our new site www.holidaycottage.ltd.uk