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.