Tonal and impulsive corrections in a BS 4142 assessment
BS 4142 allows up to +6 dB for tonality, +9 dB for impulsivity and +3 dB for intermittency. Each rests on a judgement, and that judgement is what reviewers attack.
The corrections in BS 4142 are what turn a measured specific sound level into a rating level, and they are the part of the assessment most often applied by feel and least often justified in writing. The standard allows up to +6 dB for tonality, up to +9 dB for impulsivity, +3 dB where the sound is intermittent and readily distinctive, and +3 dB for other characteristics that make the sound more noticeable.
Those are the numbers. The difficulty is that almost every one of them rests on a judgement, and a reviewing acoustician or environmental health officer will test the judgement rather than the arithmetic.
Why the corrections exist at all
A measured level says nothing about how a sound is perceived. Twelve decibels from a smooth extract fan and twelve decibels from a whining compressor with a pure tone at 250 Hz are not the same experience for the person in the garden, and BS 4142:2014+A1:2019 handles that by rating the sound rather than simply measuring it.
So the rating level is the specific sound level plus the acoustic feature corrections, and it is the rating level that is compared with the representative background sound level. A correction is therefore worth exactly as much as an equivalent change in measured level. Applying +6 dB when +2 dB was defensible can turn a scheme from acceptable into refusable on paper alone.
That is why reviewers go straight to this section.
Tonality: subjective and objective routes
The standard offers two approaches. The subjective route bands the audibility of the tone at the assessment location into three steps, with a correction of +2 dB where the tone is just perceptible, +4 dB where it is clearly perceptible and +6 dB where it is highly perceptible.
The objective route uses one-third octave or narrow band analysis and a reference method to establish tonal audibility, then converts that audibility into a correction on a sliding scale up to the same +6 dB ceiling. It is more work and far harder to argue with, which is exactly why it is worth doing on any scheme where the tone is the reason the objection exists.
| Feature | Maximum correction | Basis of judgement |
|---|---|---|
| Tonality | +6 dB | Perceptibility at the receptor, or measured tonal audibility |
| Impulsivity | +9 dB | Rapidity of onset, rate of change, level relative to the rest |
| Intermittency | +3 dB | Readily distinctive against the residual acoustic environment |
| Other characteristics | +3 dB | Attracts attention for a reason not covered above |
Two practical points. A tone that is audible at the source and inaudible at the receptor attracts no correction, because the assessment is at the assessment location. And a tone predicted from plant manufacturer data rather than measured needs the prediction method stated, since octave band sound power data smooths out precisely the narrow band content that matters.
Impulsivity and how it is judged
Impulsivity carries the largest possible correction, up to +9 dB, and the standard describes it by the rapidity of the onset of the sound, the rate of change of level, and how far the event stands above the rest of the sound. Reversing beepers, skip handling, metal on metal in a yard, bottle banks and press operations are the common sources.
Judging it well means being on site. A logged LAeq over a 15 minute period tells you nothing about onset rate, whereas a short audio recording alongside the measurement, with time weighting stated, lets a reviewer hear what you heard. That recording is the single most useful appendix in this sort of report.
Where impulsive events occur only a handful of times in the reference time interval, the honest treatment is often to assess the period containing them separately rather than smearing the correction across a whole hour.
Intermittency and the audibility test
Intermittency attracts +3 dB where the changes in the specific sound level are sufficiently readily distinctive against the residual acoustic environment to attract attention. The wording matters. The test is not whether the sound switches on and off, but whether the switching is noticeable where the receptor is.
A condenser that cycles every eight minutes in a quiet rural garden will usually earn the correction. The same condenser on a facade facing a busy distributor road may not, because the cycling disappears into the traffic. This is the correction most often applied automatically from the plant specification without anyone testing audibility at the receptor, and it is the easiest one to have struck out on review.
Adding corrections together
Corrections are cumulative in principle, and the standard does not set an arithmetic ceiling on the total. What it does expect is that you avoid double counting the same perceptual effect under two headings. An intermittent tonal fan can attract both corrections, but the impulsive onset of each start-up and the intermittency of the cycling are arguably the same phenomenon described twice, and a reviewer will say so.
A worked illustration: a rooftop chiller produces a specific sound level of 38 dB LAeq at the nearest first floor window. A clearly perceptible tone at 315 Hz gives +4 dB, and cycling that is readily distinctive in a quiet residential street gives +3 dB. The rating level is 45 dB. Against a representative background of 34 dB the difference is +11 dB, which is an indication of an adverse impact that will need mitigation rather than argument. This example is illustrative only.
Documenting the judgement
Write the correction section so that someone who was not on site can follow it. For each feature: whether it was present, where it was audible, how it was judged, the value applied and why that value and not the one above or below it. Include the survey notes, the one-third octave data where tonality is in play, and the audio recording reference.
Then state what would change the answer. If the tone is a function of one item of plant that could be specified differently, say so, because that turns the correction from a problem into a design instruction. This is also where the honest limits of the exercise belong: acoustic feature corrections carry real uncertainty, and pretending otherwise weakens the rest of the report.
How reviewers challenge a correction
The standard challenges, in the order they usually arrive:
- the correction was applied from the source specification rather than from observation at the receptor
- the subjective judgement was made by someone who never attended at the relevant time of day
- tonality was asserted without narrow band data
- impulsivity and intermittency were both applied to the same start-up behaviour
- the corrections were applied to a predicted level whose prediction method is not described
Answer all five in the report and the section stops being contentious. A Noise Impact Assessment that does this properly also makes the mitigation argument easier, because the reader can see which feature is driving the number.
Where the sound comes from kitchen extract rather than fixed plant, the acoustic work sits alongside a Ventilation and Extraction Statement, and the two should quote the same fan data. For residential development affected by existing sound, read this alongside the note on using the noise effect levels properly, which is a different framework for a different question.
A Noise Impact Assessment is a report councils expect a suitably qualified acoustician to author, and professional review is strongly advised. This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it.
Enter your postcode on PlanScope to see which reports your site is likely to need. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
What is the rating level in BS 4142?
The rating level is the specific sound level of the proposed source plus any acoustic feature corrections for tonality, impulsivity, intermittency or other characteristics. It is the rating level, not the measured level, that is compared with the representative background sound level. A correction therefore carries exactly the same weight as an equivalent change in measured level.
How large can a tonal correction be?
Up to +6 dB. The subjective route gives +2 dB where the tone is just perceptible at the receptor, +4 dB where it is clearly perceptible and +6 dB where it is highly perceptible. The objective route establishes tonal audibility from one-third octave or narrow band analysis and converts it to a correction on a sliding scale to the same ceiling.
Why is impulsivity worth up to +9 dB?
Because impulsive sound is disproportionately noticeable. The judgement rests on how rapidly the sound starts, how fast the level changes and how far the event stands above the rest of the sound. Reversing alarms, skip handling and metal on metal in a yard are typical sources, and a short audio recording is the best evidence for the value applied.
Can tonality and intermittency both be applied?
Yes, corrections are cumulative and the standard sets no arithmetic ceiling on the total. What it does expect is that the same perceptual effect is not counted twice under two headings. Applying both an impulsivity and an intermittency correction to the same plant start-up behaviour is the double count reviewers pick up most often.
Does a correction apply if the tone is only audible at the plant?
No. The assessment is made at the assessment location, so a tone that is clearly audible standing next to the unit but inaudible at the nearest window attracts no correction. Where the level is predicted rather than measured, state the prediction method, because octave band manufacturer data hides the narrow band content that creates tonality.
What evidence should the correction section contain?
For each feature: whether it was present, where it was audible, how it was judged, the value applied and why not the value above or below. Add the survey notes, one-third octave data where tonality is in play, and an audio recording reference. Say what design change would remove the correction, since that is useful to the applicant.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.