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:
make measurements of all noise at the assessment location, including the
"problem" noise, in terms of LAeq - termed the "ambient"
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
the "residual" LAeq measurement is then
(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?]
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?).
the "background" LA90 measurement is then compared against the
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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