Character corrections in a BS 4142 assessment

A character correction in BS 4142 is the penalty added to the measured or predicted specific sound level before it is compared with the background sound level. Add the corrections and you get the rating level. The rating level minus the background sound level gives the excess, and that excess is what an environmental health officer reads. A plant installation measuring 3 dB above background can become 9 dB above it on the strength of a tone nobody designed out, which is why the corrections decide more refusals than the raw measurement does.

BS 4142:2014+A1:2019 recognises four acoustic features. Three have named values. The fourth is a catch-all.

What the corrections do to the result

The method runs in a fixed order. Establish the background sound level as the LA90 over a representative period. Establish the specific sound level of the source, by measurement with the plant running and off, or by prediction from sound power data. Apply corrections for acoustic features to get the rating level. Compare.

The interpretation in the standard is expressed as a difference rather than a threshold. An excess of around 10 dB or more is likely to indicate a significant adverse impact, depending on context. An excess of around 5 dB is likely to indicate an adverse impact, again depending on context. The lower the rating level relative to background, the less likely an adverse effect becomes.

Context does real work in the standard and is frequently skipped in reports. The absolute level, the character of the area, the presence of other similar sources and the sensitivity of the receptor all bear on the conclusion.

Tonality and how it is judged

A tone is a discrete frequency component that stands out from the rest of the sound. Fan blade pass frequency, compressor whine, a transformer hum, the pure tone of a pump at running speed.

BS 4142 offers a subjective route and an objective one. Subjectively, tonality attracts 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. Objectively, one-third octave band analysis is used, with the reference method in the related international standard on environmental noise measurement, and the level difference between the tonal band and its neighbours drives the correction.

Two practical points. The subjective judgement has to be made at the receptor, not at the plant, because a tone audible beside a condenser may be inaudible 40 m away. And the judgement has to be recorded by the person who made it, with the time, the conditions and what was audible, because the number will be challenged.

Impulsivity and its correction

An impulsive sound has a rapid onset and short duration. Metal on metal, a solenoid valve, a bin lift, a door slam on a refrigerated vehicle.

The corrections are larger than for tonality: +3 dB where impulsivity is just perceptible, +6 dB where it is clearly perceptible and +9 dB where it is highly perceptible. Rapidity of onset, the level relative to the rest of the sound and the repetition rate all feed the judgement.

Impulsive features are the ones most often missed in a prediction, because manufacturer sound power data describes steady running and says nothing about the sound of the unit starting, defrosting or cycling. A predictive assessment that reports no impulsivity for a refrigeration plant with a nightly defrost cycle has assessed something other than the installation.

Intermittency and when it applies

Where the specific sound has identifiable on and off periods that are readily distinctive against the residual acoustic environment during the relevant assessment period, a correction of +3 dB applies.

The test is distinctiveness rather than duration. Plant that cycles on for eight minutes in every thirty through the night is distinctive. Plant that runs continuously is not, even though it operates all night. A unit that runs constantly during the day and cycles at night may take the correction for the night period only, which is why the assessment periods have to be handled separately rather than averaged.

The fourth feature in the standard covers other characteristics that are neither tonal nor impulsive but are readily distinctive, and it carries +3 dB. Use it rather than stretching the tonality definition to cover something that is not a tone.

The maximum correction and the judgement behind it

Adding the maximum of each feature gives a large number, and the standard expects the overall correction to be judged rather than assembled arithmetically. In practice a total above about 9 dB invites scrutiny, and a report claiming the top of every band needs to explain why.

Uncertainty belongs in the same discussion. The standard requires the assessment to address uncertainty in the measurement, the prediction and the judgement, and an assessment that reports a rating level to one decimal place while applying a subjective correction of plus or minus several decibels is presenting false precision.

Design decisions that avoid corrections

Every decibel of correction avoided is a decibel of attenuation you do not have to buy.

• Select plant with a smooth spectrum rather than relying on attenuation to remove a tone, since narrowband energy is the hardest thing to attenuate
• Use variable speed control instead of on and off cycling, which removes both the intermittency correction and the start-up impulse
• Mount plant on properly selected anti-vibration mounts, because structure-borne transmission arrives as an impulsive or tonal feature inside the dwelling
• Avoid mounting units on lightweight structures that radiate at a resonant frequency
• Where a tone is unavoidable, target the attenuator selection at that octave band rather than buying broadband performance
• Locate condensers away from reflective corners, which reinforce specific frequencies

These are mechanical design decisions taken long before an acoustician is instructed, which is the honest difficulty with BS 4142. By the time the assessment is commissioned, the corrections are usually already determined by the plant schedule.

Recording the judgement transparently

A defensible assessment shows its working on the corrections.

1. State the assessment periods and why they were chosen.
2. State the background sound level for each period with the measurement dates, positions and weather.
3. State the specific sound level and how it was derived, with the octave band data.
4. State each correction applied, the value, the route used, and the evidence for it.
5. State each correction considered and rejected, with the reason.
6. Give the rating level, the excess and the interpretation in context.
7. Address uncertainty explicitly.

Point five is the one that ends arguments. An officer who can see that intermittency was considered and rejected because the plant runs continuously will not raise it. One who cannot see it will assume it was overlooked, and a noise impact assessment that has to be revisited after submission costs more than the survey did.

For commercial kitchens and plant-heavy fit-outs the same figures reappear in the ventilation and extraction statement, so keep the two consistent. Councils usually expect a suitably qualified author for acoustic work, and professional review is strongly advised. PlanScope's professional directory is one route to a reviewer for a draft.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.