Rating level: how a BS 4142 figure is built up
The rating level in a BS 4142 noise assessment is the specific sound level plus a correction for tonality, impulsivity or intermittency, compared against the background LA90. Here is how each part is measured and where assessments go wrong.
The rating level is the number an environmental health officer looks at first. Under BS 4142:2014+A1:2019, the rating level is the specific sound level from your plant at the nearest noise-sensitive place, with a correction added for any acoustic feature that makes the sound stand out. That total is then set against the background sound level, LA90, measured over the same reference period. The difference between the two, read alongside the context of the site, carries the assessment.
This matters if you run a restaurant with a kitchen extract fan, a gym with air conditioning on the flat roof, or a small unit with refrigeration plant against a party boundary. A wrong correction, or a background measurement taken on the wrong night, shifts the answer by several decibels. That is often the distance between a condition and a refusal.
How the rating level is assembled
Three quantities do the work:
- the specific sound level, LAeq,Tr, which is the sound from the source under assessment on its own over the reference time interval
- the acoustic feature correction, a penalty in decibels for tonality, impulsivity, intermittency or other distinguishing characteristics
- the background sound level, LA90,T, which is the sound that would be there anyway with your plant switched off
Add the first two together and you have the rating level. The standard sets the reference time interval at one hour during the day, taken as 07:00 to 23:00, and 15 minutes at night, taken as 23:00 to 07:00. A night-time assessment therefore rests on a much shorter window than most operators expect, and it is the quietest quarter-hour that counts, not an average across the night.
Isolating the specific sound level
The specific sound level has to be your plant and nothing else. In practice an acoustician gets it one of three ways: by measuring with the plant running while everything else is quiet, by measuring close to the unit and calculating the level at the receptor with distance, barrier and directivity corrections, or by starting from the manufacturer's sound power data where the plant is not yet installed.
Each route has a weakness. Direct measurement at the boundary picks up road traffic and other premises unless the plant can be switched on and off. Manufacturer data is quoted under laboratory conditions and rarely includes the ductwork, the mounting or the reflecting wall behind the unit. On a shop-to-flat conversion or a new takeaway, prediction from sound power is usually the only option, so the assumptions in the calculation deserve more scrutiny than the final number.
The measurement position matters as much as the method. BS 4142 expects assessment at the nearest point where people are exposed, which for a rear extract fan is normally a first-floor bedroom window or the useable part of a garden, not the middle of the street.
Corrections for tonality, impulsivity and intermittency
The acoustic feature correction is where two competent assessors most often disagree. The standard offers a subjective route and objective methods, and the scale is broad.
| Feature | Typical correction |
|---|---|
| Tonality, just perceptible to highly perceptible | +2 to +6 dB |
| Impulsivity, just perceptible to highly perceptible | +3 to +9 dB |
| Intermittency, readily distinctive against the residual sound | +3 dB |
| Other characteristics not covered above | +3 dB |
Tonality is the whine of a fan or the hum of a compressor sitting at a definable frequency. Impulsivity covers bangs and clatters, so a bottle store or a roller shutter attracts it rather than steady plant. Intermittency applies where the source switches on and off in a way a neighbour would notice, which is exactly how a thermostatically controlled chiller behaves at two in the morning.
A correction can also be zero. If the plant produces a bland broadband hiss that merges into the residual sound, nothing is added, and assessors who apply a reflex +3 dB are misreading the standard.
Comparing the rating level with the background
The comparison is a simple subtraction, and BS 4142 attaches initial estimates of impact to the result. A difference of around +10 dB or more is an indication of a significant adverse impact. A difference of around +5 dB is an indication of an adverse impact. The lower the rating level relative to the background, the less likely an adverse effect becomes, and a rating level at or below the background points towards low impact.
Those are indications, not thresholds in policy. The Planning Practice Guidance on noise works through effect levels rather than decibel limits, and the National Planning Policy Framework asks decision-takers to avoid noise that would give rise to serious adverse impacts on health and quality of life, and to mitigate other adverse impacts. Local policy fills in the detail, which is why the wording of your council's noise policy in the adopted plan repays a read before the survey is commissioned. If you are unsure which documents carry weight on your site, our note on what counts as the development plan sets out the hierarchy.
Why the difference is not a verdict
BS 4142 devotes a full clause to context, and it is the part applicants skip. Three things modify the initial estimate: the absolute level of the sound, the character and level of the residual sound, and the sensitivity of the receptor.
A rating level 8 dB above background in an already quiet rural setting, where the absolute level is 32 dB, reads very differently from the same 8 dB excess beside an A-road where the absolute level is 58 dB. The arithmetic is identical. The conclusion is not.
Uncertainty belongs here too. The A1:2019 amendment pressed assessors to state the uncertainty in their measurements and calculations rather than presenting a rating level as an exact figure. An assessment that gives a single number with no discussion of how confident the author is in it invites a request for further information at validation.
An illustrative plant assessment at a residential boundary
The following is illustrative, not a real site.
A hot food takeaway proposes a new extract fan on the rear elevation of a terraced unit, with a residential garden and first-floor windows about 12 metres away. The operator wants to open until midnight, so the night-time period applies.
- The acoustician measures the background at the rear boundary between 23:15 and 23:30 on a midweek night and records LA90 of 38 dB.
- The fan is run and the specific sound level at the same position is LAeq,15min of 43 dB.
- The fan has an audible tone at blade-pass frequency, clearly perceptible at the boundary, so a +4 dB correction is applied.
- The rating level is 47 dB. The excess over background is +9 dB, close to the higher of the two indications in the standard.
- An in-line attenuator and anti-vibration mounts bring the specific sound level to 36 dB and remove the tone. The rating level becomes 36 dB, 2 dB below the background.
The second figure is the one worth designing for, because it gives the officer an easy recommendation and gives you a scheme that will still pass if a neighbour complains after opening.
Getting it into the application
A Noise Impact Assessment that follows BS 4142 should set out the survey dates and times, the equipment and its calibration, the measurement positions with a plan, the background and specific sound levels, every correction with the reason for it, the rating level, the context discussion and the mitigation relied on. Councils usually expect a suitably qualified author, typically a member of the Institute of Acoustics, and professional review is strongly advised before the report is submitted.
Specify the mitigation in the report rather than leaving it to a condition. Where a noise condition is imposed anyway, the discharge application will need the same evidence again, and the timing rules in our guide to deemed discharge under section 74A are worth knowing before you order plant. If the fan later moves to a different elevation, that is a change to the approved drawings, and the limits on what can be tidied up through a section 96A non-material amendment are narrower than most operators assume.
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PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.
Frequently asked questions
What is the difference between the rating level and the specific sound level?
The specific sound level is the measured or calculated sound from the plant alone over the reference time interval. The rating level is that figure with an acoustic feature correction added for tonality, impulsivity, intermittency or another distinguishing characteristic. Where the sound has no such feature the correction is zero and the two numbers are the same.
How long does the background sound level have to be measured for?
BS 4142 uses a reference time interval of one hour between 07:00 and 23:00 and 15 minutes between 23:00 and 07:00. The background LA90 must be representative of the period when the plant will operate, so an acoustician normally logs over several hours or several nights and then selects a representative typical value rather than the single lowest reading.
Can I carry out a BS 4142 assessment myself?
Nothing in planning law stops you, but councils normally expect a report from a suitably qualified acoustician with a calibrated class 1 sound level meter. Environmental health officers scrutinise the feature corrections and the choice of background period closely. A self-produced assessment without measurement equipment or competence evidence is likely to be challenged at validation or in consultation.
What correction applies to a tonal extract fan?
The standard allows +2 dB where a tone is just perceptible, up to +6 dB where it is highly perceptible, with objective methods available using one-third octave band analysis. A typical clearly audible blade-pass tone from a kitchen extract fan attracts around +4 dB. An attenuator that removes the tone removes the correction entirely.
Does a rating level below the background mean permission will be granted?
No. A low rating level makes an adverse noise effect unlikely and removes one common reason for refusal, but noise is only one material consideration. Odour, extraction ducting, hours of use, parking and highway safety are assessed separately, and BS 4142 itself says the initial estimate must be modified by the context of the site.
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.