Tonality: the reference method and the penalty applied

A tonality correction noise assessment adds decibels to the measured specific sound level before it is compared with the background, because a tone is more noticeable and more annoying than broadband sound at the same level. Under BS 4142:2014+A1:2019 the correction is between 2 dB and 6 dB depending on how perceptible the tone is, and those few decibels frequently decide whether a plant installation reads as no adverse impact or as an adverse impact requiring mitigation.

Reviewers should treat the correction as the most contestable number in the report. It is a judgement dressed as arithmetic, and the standard offers both a listening test and an objective check that the listening test can be measured against.

Why tonality is penalised separately

Human hearing separates a steady tone from the sound around it. A fan whine at 400 Hz sitting 6 dB above the surrounding spectrum is audible at a distance where the overall level has already fallen below the background, and complainants describe it as a hum or a whine rather than as noise.

BS 4142 handles this by building the rating level in two parts: the specific sound level of the source, then a character correction for tonality, impulsivity, intermittency and other distinguishing features. The rating level is what gets compared with the representative background sound level, so the correction shifts the whole conclusion rather than adding a caveat to it.

The corrections are additive where more than one character is present, which is how a compressor with a tone and an audible start-up can carry a correction large enough to change the outcome on its own.

The subjective judgement and its limits

The standard's primary route is perception. A tone that is just perceptible attracts 2 dB, clearly perceptible 4 dB, and highly perceptible 6 dB. The assessor listens at the receptor position and records the judgement.

Three weaknesses follow, and a reviewer should look for each.

The judgement is made at one place and time. A tone audible at a first floor bedroom window on a still night may be inaudible at the ground floor position the assessor used in the afternoon.

The assessor's hearing varies, and so does their expectation. Someone who has spent the morning beside the plant is poorly placed to judge audibility at 30 metres.

And the tone may not be running. Condenser fans cycle, and a survey that happens to catch the low speed mode records a different character from the one the neighbours hear at 2am. Survey duration matters here for the same reasons set out in our note on how long a background noise survey should run.

The one third octave reference method

BS 4142 gives a simplified objective method for identifying whether a tone is likely to be audible. Measure the one third octave spectrum at the receptor, then compare each band with the two bands either side. A tone is indicated where the band level exceeds both adjacent bands by:

The thresholds narrow as frequency rises because the ear's critical bands are wider at low frequency, so a low frequency tone has to stand further above its neighbours before it separates out.

This method tells you whether a tone is present. It does not tell you how perceptible it is, and the standard is explicit that it is a simplified check rather than a full analysis. Where the answer matters, the fuller reference methods in BS ISO 1996-2, including tone to noise ratio, prominence ratio and the Joint Nordic method for tonal audibility, give a quantified audibility figure from which a correction can be derived.

Turning audibility into a correction

The awkward step is the conversion. The simplified method produces a yes or no. The subjective scale produces 2, 4 or 6 dB. The Joint Nordic method produces a tonal audibility value and an associated correction on a continuous scale up to 6 dB.

A defensible report does three things. It states which method was used and why. It reconciles the objective result with the subjective judgement, explaining any disagreement rather than quietly adopting the lower correction. And it reports the residual sound spectrum as well as the specific sound spectrum, because a tone that is present in both is a feature of the area rather than of the proposal.

Illustrative example: a roof mounted air source heat pump serving a small office, measured at the nearest residential facade at 38 dB LAeq,15min specific sound level. The 250 Hz band stood 11 dB above both neighbours, which exceeds the 8 dB indicator for that band. The assessor recorded the tone as clearly perceptible, giving a 4 dB correction and a rating level of 42 dB against a background of 39 dB LA90. A 3 dB excess sits in the range BS 4142 treats as an indication of an adverse impact depending on context. The figures are illustrative and used to show the method.

That example shows why the correction is worth arguing about. Without it the rating level would have sat below background.

Tonal sources that recur on development sites

The same short list produces most tonal complaints:

1. Axial and centrifugal fans, where the blade passing frequency produces a tone at the number of blades multiplied by the rotational speed.
2. Refrigeration and air conditioning condensers, including the cycling behaviour that also triggers an intermittency correction.
3. Air source heat pumps, increasingly on small commercial and residential schemes.
4. Transformers and substations, with a low frequency hum at twice mains frequency and its harmonics.
5. Pumps, compressors and gear driven plant, often with structure-borne transmission as well as airborne.
6. Kitchen extract fans, which combine a tone with the odour and grease issues covered in our note on commercial kitchen extraction.

Structure-borne transmission deserves a separate look. A tone reaching a flat through the building frame will not appear in an external measurement at all, which is why an assessment for plant on or inside a building needs internal positions as well.

Designing the tone out at source

Correcting a tone is more effective than attenuating a broadband spectrum, because the energy is concentrated.

Plant selection comes first. A larger fan running slowly produces a lower blade passing frequency at a lower level than a small fan working hard, and the low frequency tone is easier to break up with a plenum. Changing the number of blades moves the tone to a frequency where attenuation is easier or perception is lower.

Then the mounting. Anti-vibration mounts and resilient supports address structure-borne paths, and a transformer on a rigid slab transmits a hum that no acoustic enclosure will fix.

Tuned attenuators, plenum linings and reactive silencers work where the tone frequency is known and stable. Where the plant is variable speed, the tone moves, and broadband treatment plus enclosure becomes the safer approach.

Reporting so a reviewer can check the work

Councils normally expect a Noise Impact Assessment to come from a suitably qualified acoustician, and professional review is strongly advised because the character corrections are where the judgement sits. A reviewer needs enough in the report to reach their own view, which means:

• One third octave spectra for the specific sound, the residual sound and the background, at each measurement position.
• Time histories long enough to show cycling, with the plant operating mode recorded.
• Measurement positions described by height and distance, with photographs.
• Instrument class, calibration before and after, and any corrections applied.
• The tonality method used, the numerical result, the subjective judgement and how the two were reconciled.
• The rating level calculation set out term by term so each addition can be checked.

Where the assessment supports a condition, the condition should be written against the rating level at a defined position rather than against a plant specification, so that a substituted unit still has to meet the limit. That principle is the same one discussed in our note on what a vibration condition can require.

This is general guidance, not legal advice, and the assessment of any particular source depends on its spectrum, its operating pattern and the acoustic context, so check the position with your local planning authority and a qualified acoustician before relying on it. PlanScope produces AI-assisted drafts structured to UK planning policy, with the measured data left for your acoustician to supply and verify. Enter your postcode to see which reports your site needs.

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