Rating level and character corrections under BS 4142
The rating level is where most BS 4142 assessments are won or lost, because the acoustic feature corrections are a matter of judgement that has to be evidenced. Here is how to set them out.
The BS 4142 rating level is the specific sound level plus any corrections for the character of the sound, and it is the number that gets compared with the background. Everything contentious in an assessment under BS 4142:2014+A1:2019 sits in those corrections, because they are matters of judgement rather than measurement. A reviewer reading your report is checking three things: that the specific sound level relates only to the source under investigation, that the background is representative, and that every correction you applied or omitted is justified in writing.
What the rating level is made of
The chain is short. The specific sound level is the equivalent continuous A-weighted sound pressure level of the source under investigation at the assessment location, over the reference time interval. Add the acoustic feature correction and you have the rating level. Compare that with the background sound level and you have the initial estimate of impact, which context then modifies.
The reference time intervals are fixed by the standard: one hour during the day, from 07:00 to 23:00, and fifteen minutes at night, from 23:00 to 07:00. Choosing a longer averaging period to smooth out a noisy plant cycle is a common error and it invalidates the comparison.
Assessment locations matter as much as periods. Outside a dwelling, that usually means a position 1 m from the facade at the relevant window, or a free-field position at least 3.5 m from any reflecting surface other than the ground, with the conversion between the two stated rather than assumed. Where internal levels are assessed, the standard expects the window condition to be specified.
Measuring the specific sound level
Isolating the source is the hard part. On a working site the plant you are assessing runs alongside road traffic, other plant, and activity that belongs to the residual sound rather than to the specific sound.
Three approaches, in descending order of confidence:
- Measure with the source on and off under otherwise similar conditions, and derive the specific sound level by logarithmic subtraction, stating the correction applied
- Measure close to the source and calculate the level at the assessment position, with the propagation model, distance, barrier and directivity assumptions set out
- Calculate from manufacturer sound power data where measurement is impossible, and say plainly that the assessment rests on declared data
Record the instrument class, the calibration check before and after, the windshield, the weather during measurement and the reasons for excluding any period. Our note on unattended noise monitoring duration, position and validity covers the longer-term measurements that support this work.
Manufacturer data has a habit of describing a unit in free field at full speed with no ducting. Real installations have bends, grilles, resonating support frames and a control strategy that runs the fan hard on a summer night. Say which you used.
Establishing the background sound level
The background sound level is the A-weighted sound pressure level exceeded for 90 per cent of the measurement period, measured when the specific source is not operating, over periods representative of the assessment intervals.
Representative is the word that causes arguments. Taking the single lowest fifteen-minute value from a week of data produces an artificially low background and inflates the excess. Taking the arithmetic mean of everything hides the quiet hour when complaints actually arise. Present the distribution, state the value you have adopted, and explain why. A table of the lowest few values alongside the modal value lets a reviewer follow the reasoning.
Watch for contamination. A background period that includes a passing vehicle, a dog, or the source itself running at low load is not a background period. And on a site where the source already operates continuously and cannot be switched off, the standard's method has to be adapted, which is a limitation to declare rather than a problem to hide.
Acoustic feature corrections and how to justify them
BS 4142 provides for corrections where the sound has characteristics that make it more noticeable at the receptor. The standard sets out three named features with graduated values, plus scope for other characteristics.
| Feature | Basis for the correction | Range in the standard |
|---|---|---|
| Tonality | Subjective method, or the reference method based on one-third octave analysis | 2 dB just perceptible, 4 dB clearly perceptible, 6 dB highly perceptible |
| Impulsivity | Rapidity of onset, level and prominence of the impulsive element | 3 dB, 6 dB or 9 dB by prominence |
| Intermittency | Readily distinctive on and off behaviour against the residual sound | 3 dB |
| Other characteristics | Reasoned judgement, described in the report | Judged and stated |
Justify each one with evidence rather than a sentence. For tonality, show the one-third octave spectra and identify the tone and its frequency. A 250 Hz tone from a condenser fan is audible in a bedroom in a way that a broadband level does not capture. For impulsivity, a time history at fast time weighting shows the onset. For intermittency, describe the duty cycle in the operating hours being assessed.
Corrections are not additive without thought. Where a sound is both tonal and intermittent the standard expects a judgement about the combined character, and a report that stacks the maximum of every category invites a challenge. So does a report that applies none at all to a rooftop chiller.
The excess and what it indicates
The comparison is rating level minus background sound level, and the standard describes the indications that follow. A difference of around +10 dB or more is an indication of a significant adverse impact, depending on context. A difference of around +5 dB is an indication of an adverse impact, depending on context. The lower the rating level relative to the background, the less likely the sound is to be an indication of an adverse impact; where the rating level does not exceed the background, that is an indication of a low impact, again depending on context.
Note the repetition of that final phrase. The excess is an indication, not a verdict, and every one of those statements in the standard is qualified.
Context, and why the standard insists on it
The standard requires the initial estimate to be modified by context, and sets out what context covers: the absolute level of the sound, the character and level of the residual sound compared with the specific sound, and the sensitivity of the receptor including any existing mitigation such as glazing and acoustic ventilation.
Two worked contrasts make the point. An excess of +6 dB where the absolute rating level at the facade is 32 dB at night, in a dwelling with sealed glazing and mechanical ventilation, is a different proposition from +6 dB at 52 dB with openable windows as the only ventilation. And a specific sound that is tonal against an otherwise quiet rural background is more intrusive than the same excess against a residual sound of similar character.
Write the context section as analysis. Listing the three factors and concluding that the impact is acceptable is the pattern that gets assessments sent back by environmental health.
Presenting the assessment so a reviewer can follow it
A reviewer should be able to reproduce your rating level from the report alone. That means the measurement and calculation data in tables, not in prose, and a clear statement of every assumption.
- Measurement positions on a plan, with heights, distances to facades and photographs
- Instrumentation, calibration records and the measurement periods with dates and weather
- Specific sound level derivation, including any on and off subtraction or propagation calculation
- Background data with the distribution and the adopted value, and the reasoning
- Each acoustic feature correction with its supporting evidence, including spectra
- The rating level, the background, the excess, and the context discussion as separate items
- Any mitigation assessed, with the assumed insertion loss and its source
Where the Noise Impact Assessment will be read alongside a licensing application, keep the planning and licensing regimes separate in the reporting, as our note on planning control and licensing control of entertainment noise explains. Reviewers who sign off drafts can be approached through the PlanScope professional directory.
PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required.
Frequently asked questions
What is the difference between the specific sound level and the rating level?
The specific sound level is the A-weighted equivalent continuous level of the source under investigation at the assessment position over the reference time interval. The rating level is that figure plus any acoustic feature correction for tonality, impulsivity, intermittency or other characteristics. Only the rating level is compared with the background sound level.
What reference time intervals does BS 4142 use?
One hour during the day, taken as 07:00 to 23:00, and fifteen minutes at night, taken as 23:00 to 07:00. Using a longer averaging period to smooth out plant cycling is a common error, because it lowers the apparent specific sound level and breaks the comparison the standard is built on.
How large can an acoustic feature correction be?
The standard provides graduated corrections of 2, 4 or 6 dB for tonality according to perceptibility, 3, 6 or 9 dB for impulsivity according to prominence, and 3 dB where the sound is readily distinctive as intermittent. Other characteristics can be corrected by reasoned judgement, which must be described and justified.
Which background value should be adopted from a week of data?
One that is representative of the period being assessed, not the single lowest value and not a mean of everything. Present the distribution, show the lowest few values alongside the modal value, state the figure adopted and explain the reasoning. Exclude periods contaminated by the source or by atypical events.
Does an excess of 10 dB mean the application will be refused?
No. The standard treats a difference of around 10 dB as an indication of adverse impact at the upper end of its scale, depending on context. Context covers the absolute level, the character of the residual sound and the sensitivity of the receptor, including existing glazing and ventilation. The assessment has to work that through rather than stop at the arithmetic.
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.