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A brief explanation of the most widely used UK standard for assessment of environmental noise - similar but not identical to European Standards.

The document is British Standard 4142 and its title is "Method for Rating Industrial Noise Affecting Mixed Residential and Industrial Areas". The standard is very complex and if you wish to use it, a copy of the latest version will be required. The main points of the standard are [in brief] as follows:

  1. make measurements of all noise at the assessment location, including the "problem" noise, in terms of LAeq - termed the "ambient" noise level

  2. a measurement is then made of all the noise excluding the "problem" noise in terms of both LAeq and LA90; these measurements are termed the "residual" and "background" noise levels respectively.

  3. the "residual" LAeq measurement is then subtracted (logarithmically) from the "ambient" LAeq measurement to produce the noise level produced by the "problem" noise alone - termed the "specific" noise level [are you following this?]

  4. if the "problem" noise is tonal [containing a noticeable hiss, whine or hum] or if it is impulsive [contains bangs clatters, clicks or thumps] or if it is irregular enough to attract attention [?] a correction of 5 dBA is added to the "specific" level to produce the "rating level" (still with it?).

  5. the "background" LA90 measurement is then compared against the "rating" level.

  6. If the "rating" level exceeds the "background" by around 10 dBA or more this "indicates that complaints are likely". A difference of around 5 dBA is of marginal significance; at a difference below 5 dBA, the lower the value, the less likely that complaints will occur; a difference of -10 dBA or more is "a positive indication that complaints are unlikely".

Such measurements are made out of doors, away from any reflecting facades, but still near to the dwellings within which residents may be affected. Some ideas for instrumentation appear on this site.

During the daytime, measurements of the "problem" noise are averaged over an hour, whereas at night the average is performed over five minutes. The standard should not be used where BOTH the background noise and rating level are low i.e. the background should not be below 30 LA90 AND the rating level should not be below 35 dB.

The standard requires that the weather conditions are recorded and are such that the weather does not cause spurious measurements, e.g. the wind speed at the microphone can sometimes cause a "fluting" effect across the microphone grid; given that the sound level meter cannot differentiate (not yet) between noises, it will measure the fluting effect.   Windmuffs/windshields are used to reduce this effect, but depending on the loudness of the "real" noise, fluting can be important when the wind speed at the microphone is as low as 5 m/s.  You should check on recommended limits with the meter manufacturer if this issue is important.  In addition, this standard and many others require that the measurements are made under representative conditions, i.e. not when the weather conditions produce real, but untypical, variations in the level of noise.  See the section on Weather Effects for more detail.

You will need to obtain a copy of the latest version of the standard, should you intend to use BS4142 in "real life". The Foreword of the Standard states "It should be noted that noise assessment is a skilled operation and should be undertaken only by persons who are competent in the procedures". If, having read the above and the standard, you don't feel competent you can either consult the Directory for a list of consultants or for training in Environmental Noise Assessment.

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