BS 4142 rating penalties: tonality and impulsivity

The rating penalty is where most BS 4142 assessments are won or lost. The measured or predicted specific sound level rarely moves by more than a decibel between two competent assessors, but the acoustic feature corrections can differ by 9 dB, and 9 dB is the difference between a conclusion of no adverse impact and a conclusion of serious harm to residential amenity. That is why environmental health officers read the correction paragraph first.

BS 4142:2014+A1:2019 asks you to convert a specific sound level into a rating level by adding corrections for the character of the sound. Character, not level. A quiet tone can be more intrusive than a louder broadband hum, and the standard exists to capture that.

What the rating level is and how it is built

The specific sound level is the equivalent continuous A-weighted sound pressure level of the source under assessment, at the assessment location, over the reference time interval: one hour during the day and 15 minutes at night. The rating level is that figure plus the acoustic feature corrections.

The rating level is then compared with the background sound level, the LA90 over the same periods. The standard describes a difference of around +10 dB or more as an indication of a significant adverse impact and a difference of around +5 dB as an indication of an adverse impact, with context modifying both.

So the corrections do not sit at the margin of the assessment. They move the rating level directly, and a +6 dB tonal correction on a source predicted at 4 dB above background takes the outcome across the threshold the officer is reading against.

Tonality and the three assessment routes

A tone is a discrete frequency component that stands out from the rest of the spectrum. The standard offers three ways to establish one, and they are not interchangeable.

The subjective route is a trained listener judging perceptibility at the assessment location, with a correction of +2 dB where a tone is just perceptible, +4 dB where it is clearly perceptible and +6 dB where it is highly perceptible. It is legitimate and it is the weakest evidence, because nobody can check it later.

The one-third octave band route compares the level in a band with the levels in the two adjacent bands. The standard treats a tone as present where the difference exceeds 15 dB in low frequency bands, 8 dB in the middle bands and 5 dB in the high frequency bands, and the magnitude of the exceedance informs where in the +2 to +6 dB range the correction sits.

The reference method is the objective tonal audibility procedure in BS EN ISO 1996-2, which produces a tonal audibility value and a corresponding correction. It takes narrow-band analysis and more work, and it is the route to use where the outcome is contested or where the source is a fan, a transformer or a compressor with a known blade or mains frequency.

Use the most objective route you can afford, and record which one you used. A reviewer cannot test a correction whose method is not stated.

Impulsivity and how the correction is judged

Impulsive sound has a rapid rise time and short duration: reversing alarms, metal on metal in a yard, skip handling, a press or a hammer. The corrections are larger than for tonality, at +3 dB where impulsivity is just perceptible, +6 dB where it is clearly perceptible and +9 dB where it is highly perceptible.

Judgement here is more subjective than most reports admit. The standard points to the rate of change of level, the overall level of the impulse relative to the residual sound, and how often the impulses occur. Low frequency content complicates this further, and our note on low frequency noise complaints explains why those cases resist the standard method. A useful supporting approach is to record the time history at a fine logging interval and show the peaks against the underlying level, so that a reader can see what the assessor heard.

Where impulsive events are rare, the reference time interval matters enormously. One skip lift in a 15-minute night-time period is a different assessment from continuous impact noise across an hour, and choosing the interval to avoid the event is the sort of thing reviewers look for.

Intermittency and when it attracts a penalty

Intermittency is about sound that switches on and off in a way that is readily distinctive against the residual acoustic environment. Where it is, the standard supports a correction of +3 dB.

The trap is treating any cyclical source as intermittent. A chiller that cycles on and off every 20 minutes at a level well above the residual sound is intermittent in the sense the standard means. A refrigeration unit running continuously with a slowly varying load is not, even though its level moves.

Ask whether a resident would notice the transitions. That is the question the standard is really posing, and it is why the assessment location and the residual level matter as much as the source itself.

Other characteristics and subjective judgement

The standard allows a further correction, typically +3 dB, for other characteristics that make the sound more noticeable, which in practice usually means a distinctive sound quality that is not captured as tonal or impulsive. Speech and music are the common examples on mixed-use sites.

Corrections are cumulative but should not be applied mechanically. Where a source is both tonal and impulsive, add both, then step back and ask whether the resulting rating level represents the sound a resident would experience. If it does not, say so and explain the judgement rather than hiding it.

This is the honest uncertainty in the method. Two competent acousticians can reach rating levels 4 dB apart on the same plant, and pretending otherwise damages the profession's credibility in front of a planning committee.

Presenting the correction so an officer can test it

A Noise Impact Assessment should show the corrections in a table alongside the evidence for each one: the method used, the measured or predicted values that support it, and the correction applied. Put the one-third octave spectrum in the report, not in an appendix nobody opens.

State the reference time intervals and why they were chosen. Include the time history plots. Where a subjective judgement was made, say who made it, when, in what conditions and from where.

Then run the sensitivity. Show the outcome with and without the contested correction, so that the officer can see whether the conclusion depends on it. Where it does, that is the point at which mitigation should be designed in rather than argued about, and where a Ventilation and Extraction Statement or an attenuator specification earns its place.

Where reviewers most often disagree

Four recurring disputes. A tonal correction asserted subjectively with no spectrum to support it. An impulsive correction omitted entirely on a yard with reversing vehicles. A reference time interval chosen to dilute short events. And corrections applied to a predicted specific sound level that was itself derived from manufacturer data without an allowance for installed conditions.

The last one is the quiet killer. Manufacturer sound power data is measured under controlled conditions, and a unit installed in a reflective yard against a boundary wall behaves differently. State the assumptions, and where possible verify them after installation through a condition requiring a post-completion test.

Assessments under BS 4142 are prepared by acousticians, and councils expect a suitably qualified author. PlanScope drafts the report structure and policy framing around your measurements, and the draft should go for professional review before it is submitted.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.