Background sound level and how a survey establishes it
Background sound level is the A-weighted level exceeded for 90 per cent of an interval. Most disagreements in a BS 4142 assessment come down to how it was established.
The background sound level survey is the measurement that everything in a BS 4142 assessment is compared against, and it is the part most often challenged by an environmental health officer. Background sound level is defined as the A-weighted sound pressure level exceeded for 90 per cent of a given time interval, written as LA90,T. It describes the quiet underneath the noise: the level that persists when the lorry has gone and the dog has stopped.
Set it too low and the proposed plant looks unacceptable. Set it too high and the assessment is not believed. Most disagreements about industrial and commercial sound come down to this single figure.
What the level is describing
LA90 is a statistical index rather than an average. Over a measurement interval the sound level fluctuates, and LA90 is the level exceeded for 90 per cent of that interval. Brief loud events barely move it, which is the point: it captures the underlying sound climate against which a new continuous source will be heard.
That is different from the ambient level, LAeq, which includes everything. It is also different from the residual level, which is the ambient sound remaining when the specific sound source under assessment is removed.
A reviewer should check that the report uses the three terms correctly and does not slide between them. Reports that quote a background level derived from an LAeq measurement have made an error that changes the conclusion.
Measurement indices and periods
BS 4142:2014+A1:2019 sets reference time intervals of one hour during the day, taken as 07:00 to 23:00, and 15 minutes at night, taken as 23:00 to 07:00.
The shorter night-time interval matters. A 15-minute reference period responds to short quiet spells that an hourly interval would smooth over, so night-time background levels are often markedly lower than a casual reading of the daytime data would suggest. On a site with a plant running 24 hours, the night-time background is almost always the governing case.
Report the measured LA90 for every reference interval, not a single figure for the whole survey. The distribution matters, and a reviewer should ask for the time history if only summary values are given.
Choosing representative days and times
The survey has to cover periods representative of when the specific sound source will operate and when the receptor is sensitive.
For a restaurant extraction fan running until midnight, that means evening and late-evening measurements on the days the business trades, including a weekend. For an industrial process running continuously, it means weekday and weekend, day and night. For a site whose sound climate is dominated by a road, it means avoiding school holidays and, where practical, avoiding periods when the road is subject to works or diversions.
Duration is a judgement. Longer is better, and unattended logging over several days gives a far stronger dataset than two attended hours, but unattended measurement cannot tell you what caused any given event. The usual compromise is unattended logging over a representative period with attended observation during part of it, and the report should say which periods were attended.
Excluding atypical events
This is where honest practice and convenient practice diverge.
Legitimate exclusions are events unrelated to the normal sound climate: rainfall, wind speeds above about 5 m/s at the microphone, nearby construction or road works, an aircraft overflight during an attended sample, birdsong dominating a dawn interval in a way that would not persist.
The rule a reviewer should apply is that exclusions must be identified, justified and quantified. A report that excludes intervals without saying which, why or what effect the exclusion had on the selected background level has removed the reader's ability to check the most consequential decision in the survey.
Watch for the opposite error too. Excluding every interval that produced a low background, on the grounds that it was unusually quiet, biases the figure upwards and suits the applicant.
Measurement position and height
Measure free-field where you can: at least 3.5 m from any reflecting surface other than the ground, at a height of 1.2 m to 1.5 m above ground level for a ground-floor receptor.
Where a free-field position is impossible and measurement has to be taken at a facade, the report must apply the appropriate correction and say that it has. Failing to correct a facade measurement overstates the background by roughly 3 dB, and that alone can move an assessment from an adverse impact to a negligible one.
Position matters as much as height. The microphone should be where the receptor actually is, or at a location the report demonstrates is representative of it. A background level measured in a sheltered car park behind a building is not representative of a garden facing the source.
Record the position with a plan, a grid reference and a photograph. A reviewer cannot verify a measurement position described in words alone.
| Element | What the report should state |
|---|---|
| Index | LA90,T with the reference interval used |
| Reference intervals | One hour daytime, 15 minutes night-time |
| Position | Free-field or facade, distance from reflecting surfaces, height |
| Duration | Survey dates, attended and unattended periods |
| Exclusions | Which intervals, why, and the effect on the selected level |
| Instrumentation | Class 1 sound level meter, calibration before and after, drift, laboratory certificate date |
| Weather | Wind speed and direction, rainfall, temperature |
Uncertainty and how it is reported
BS 4142 expects uncertainty to be considered and its effect on the conclusions discussed. Sources include instrument tolerance, the representativeness of the measurement period, the measurement position, meteorological variability and, for a proposed source, the accuracy of the source data.
A reviewer should look for a proportionate discussion rather than a single quoted figure. Where the difference between the rating level and the background level sits close to a threshold that changes the conclusion, uncertainty is not a footnote. It is the finding.
The honest position on many surveys is that the background level is known to within a couple of decibels, and that a conclusion resting on a 1 dB margin is not supported by the data. Saying so is better practice than implying a precision the method does not deliver.
Agreeing the survey with environmental health
Agree the method before the survey rather than defending it afterwards. A short note to the environmental health officer setting out proposed positions, dates, durations, reference intervals and the approach to exclusions will usually get a response within a couple of weeks, and it removes the most common ground for rejecting a submitted assessment.
Include the receptors you propose to assess and why. Officers sometimes add one you had not considered, and adding it before the survey costs nothing.
Where the background level is being established for a scheme that also needs internal criteria, the two surveys can often share instrumentation and site visits, though the indices and periods differ. A Noise Impact Assessment covering both should be explicit about which data supports which conclusion.
Reviewers asked to sign off a draft should check the survey design before the arithmetic. An assessment built on an unrepresentative background level cannot be rescued by correcting the rating penalties. The professional directory is the route for review and sign-off, and browsing the report types by category shows where noise evidence sits alongside the rest of an application.
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, and a BS 4142 assessment should be carried out by a suitably qualified acoustician.
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 background, ambient and residual sound?
Background sound level is the LA90, the level exceeded for 90 per cent of the interval. Ambient sound level is the LAeq, the total sound from all sources. Residual sound is the ambient sound remaining once the specific source under assessment is removed. Reports that confuse the three reach the wrong conclusion.
What reference time intervals does BS 4142 use?
One hour during the day, taken as 07:00 to 23:00, and 15 minutes at night, taken as 23:00 to 07:00. The shorter night-time interval responds to brief quiet spells that an hourly interval smooths over, so night-time background levels are usually lower and often govern the assessment for continuously operating plant.
When is it acceptable to exclude measurement periods?
When the period was affected by something unrelated to the normal sound climate: rainfall, wind above about 5 m/s at the microphone, nearby construction or road works, or a one-off event during an attended sample. Exclusions must be identified, justified and their effect on the selected level quantified.
Does it matter whether the measurement was free-field or at a facade?
Considerably. A facade measurement includes reflected sound and needs correcting, and failing to correct it overstates the background by around 3 dB. That margin alone can move an assessment between conclusions, so the report must state which position was used and confirm the correction applied.
How should uncertainty be handled?
Proportionately and honestly. BS 4142 expects uncertainty to be considered and its effect on the conclusions discussed. Where the margin between the rating level and the background level is within a decibel or two of a threshold that changes the outcome, the uncertainty is the finding rather than a caveat.
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.