BS 4142:2014 Methods for Rating and Assessing Industrial and Commercial Sound is
a British Standard that describes a method for assessing the impact of a proposed
or existing industrial or commercial sound source. Its principal use is therefore
either to assess noise from new or changed industrial or commercial premises, to accompany a
planning application, or to assess noise which may be giving rise to
complaints. In respect of both of these, a potential or existing noise-maker is
likely to be told �you need a BS4142 assessment�.
Essentially, a BS 4142 assessment is a means of
�rating� noise impact based on a comparison between the noise from the source
which is being assessed (the specific noise) and the background noise which
would exist in the absence of that source. The specific noise is �corrected�
for �acoustic features� by adding a decibel penalty for tonal or impulsive
content and for any other features which are readily distinctive against the background
acoustic environment. The corrected noise level is referred to as the �rating
level� and is intended to reflect people�s general reaction to noise whereby
noise with such features is found to be more annoying than featureless noise at
the same level. This rating level also includes a similar correction applied where
the noise source does not operate continuously.
It may be necessary to measure the specific
noise at a time when other noise is at a minimum to obtain a representative
picture of what its true level is. Sometimes it is necessary to allow for such
other noise by determining its contribution to the measured source noise and
correcting the measured noise level accordingly. This helps to build a true
picture of the specific noise, uncorrupted by other sources. The specific noise
is quantified using the dB LAeq noise measurement index. This is effectively
an average of the noise over the measurement period.
Unlike source noise, the background noise is
quantified using the dB LA90 noise measurement index. This is the
noise level exceeded for 90% of the measurement period and, whilst it is above
the minimum level, represents the steady noise which exists in the absence of any
other sources. In order to quantify the background noise, measurements have to
be made without the source noise operating or, alternatively, at a location
where the noise environment is unaffected by the source noise but otherwise the
same as where the source noise would be heard.
Normally it is necessary to make repeated
measurements of the background noise as it can vary considerably, particularly
between day and night and even between different points of the day. It may also
vary with wind direction and other meteorological factors. As a result,
measurement equipment may need to be left in a secure environment,
representative of the location of interest, to log successive measurements of
the dB LA90 value. BS 4142 says that the measurement interval should
not be less than 15 minutes and that the objective is to quantify what is
typical during particular time periods. It provides an example of statistical
analysis whereby the �modal� value (the level most commonly occurring) is
considered �typical�. In order to obtain the maximum amount of data for
statistical analysis, the shortest period allowed under BS4142 should be used, and
therefore such measurements are usually made using a 15-minute measurement
interval.
Once the specific noise level, as
corrected for acoustic features, and typical background noise level, relevant
to periods of operation of the source, have been established, the two are
compared. BS4142 states that:
a) Typically, the greater this
difference, the greater the magnitude of the impact.
b) A difference of around +10
dB or more is likely to be an indication of a significant adverse impact,
depending on the context.
c) A difference of around +5
dB is likely to be an indication of an adverse impact, depending on the
context.
d) The lower the rating level
is relative to the measured background sound level, the less likely it is that
the specific sound source will have an adverse impact or a significant adverse
impact. Where the rating level does not exceed the background sound level, this
is an indication of the specific sound source having a low impact, depending on
the context.
The key
here is that professional judgement is required from an acoustic consultant as something which might be indicative of a high impact
under some circumstances may not be so under others!
BS 4142 states
that, �for a given difference between the
rating level and the background sound level, the magnitude of the overall
impact might be greater for an acoustic environment where the residual sound
level is high than for an acoustic environment where the residual sound level
is low�. In particular, �where
background sound levels and rating levels are low, absolute levels might be as,
or more, relevant than the margin by which the rating level exceeds the
background�. This essentially means that where specific noise rating levels
are low, for example less than 35 dB LAeq , there may be no need for
a background noise survey to demonstrate a low impact.
Conversely,
it also states that �where residual sound
levels are very high, the residual sound might itself result in adverse impacts
or significant adverse impacts, and the margin by which the rating level
exceeds the background might simply be an indication of the extent to which the
specific sound source is likely to make those impacts worse�.This means that in some very noisy
areas, there may be a requirement for specific noise rating levels to be at
least 10 dB below the level of existing noise.
The methodology
contained in BS 4142 initially appeared in the �Wilson Report� of 1963(!) at
Appendix 15, Simplified Procedure for
Assessing Reaction to Industrial Noise in Mixed Residential and Industrial
Areas. The Wilson Report itself, which comprises the final report of the
Committee on the Problem of Noise, is worthy of note as it marks the start of
awareness of noise as a significant environmental problem. Interestingly, in
this first methodology, the specific noise level is assessed against the level
of noise which could be expected in a given environment, with various modifiers
including the time at which it occurs and the type of area, rather than the
background noise which was introduced in the first British Standard version in
1967.
BS 4142:1967,
Method of Rating Industrial Noise
Affecting Mixed Residential and Industrial Areas, clearly shares a similar
title to the 1963 Wilson Report Appendix but it specifies a comparison between
the specific noise level and the measured background noise level, which was
noted to be the noise level exceeded for 90% of the time, or a �notional�
background noise level which was based on the character of the area. The
correction for tonal or impulsive noise was carried through from the Wilson
report and the standard remained current until 1990.
BS 4142:1990
shared the same title except that it was called �method for rating��
rather than �method of rating��. The significance of this change is not
explained! The methodology did not change fundamentally but more detail was
included to bring it up to date, the concept of a �notional� background noise
level was dropped and reference was made to the use of sound level meters to
measure the background noise level directly, as an LA90, and the
specific noise as an LAeq, both of which had become possible using
portable sound level meters in the intervening period.
BS 4142:1997
continued with the same title and methodology as used in 1991 but a fundamental
flaw in the 1990 standard was corrected, where the measured specific noise was
corrected for extraneous �other� noise (referred to as residual noise) by
subtracting the LA90 value of this noise rather than the LAeq.
Most noise professionals simply carried out this procedure correctly during the
intervening years but we can be sure that advocates at Public Inquiry, and in
court, would have had some fun with this until it was corrected.
BS 4142:2014
contained some significant updates to the 1997 version including a title (and
content) which replaces the word �noise� with �sound�, introduces the concept
of commercial sound and overhauls the acoustic character corrections (+5 dB for
tonal and impulsive content and where the specific noise is irregular enough to
attract attention) which had remained unchanged since the 1963 appendix!
The
current version of the standard is currently under review as to what further updates
are required, e.g. whether the character corrections can actually be added
together (one of the changes in the 2014 version) and whether it is valid to
subjectively assess the characteristic impact rather than have it all based on
measurement.
What
is clear from recent debate on the content of the latest version is that the
final judgement on impact is fundamentally dependant on context and
professional judgement. This echoes the original premise of the Wilson Report
Appendix which was that the context of the noise; such as whether it was from a
new factory, a factory which had been in existence for a few years in an
atypical environment, or from an old established factory in character with the
area; and the type of district it was in (rural through residential urban to
heavy industrial) was fundamental to the assessment.
Certainly,
one of the things which is increasingly being debated is whether the
methodology as written has the precision which one might expect from the a
standard. Certainly it varies greatly from the standards which are used to
govern such things as sound insulation tests and acoustic testing of wind turbines.
Please
get in touch with one of our expert acoustic consultants if you have been told
you need a BS4142, or any other noise assessment of industrial or commercial sound.