Unattended noise monitoring: duration, position, validity
An unattended noise survey succeeds or fails on duration, microphone position and how honestly you handle weather-affected data. The practical rules, and what belongs in the appendix.
An unattended noise monitoring survey stands or falls on three decisions made before the meter leaves the office: how long it runs, where the microphone sits and what you will do with data the weather ruined. Get those right and the results survive a consultee review. Get them wrong and an environmental health officer will ask for the survey again, usually at the point in the programme where you have least slack.
Unattended monitoring means the meter logs continuously with nobody present. It suits background sound measurement over several days. It does not suit source identification, because a logger cannot tell you that the 62 dB event at 02:14 was a reversing alarm rather than a dog.
Choosing between attended and unattended surveys
Most surveys need both. The unattended logger establishes the baseline across full days and nights. Short attended periods, with the engineer listening and taking notes, identify what makes up that baseline and capture the character of any specific source.
An assessment under BS 4142:2014+A1:2019 depends on both. The background sound level is a statistical measure taken from logged data, while the character corrections for tonality, impulsivity and intermittency come from someone standing there hearing it. Submitting logged data alone, with character corrections asserted rather than observed, is one of the more common reasons a Noise Impact Assessment comes back with questions.
Survey duration and representative days
There is no universal minimum, and anyone quoting one is usually reciting a local habit. The test is whether the measured period covers the conditions the assessment relies on.
In practice that means covering the periods your assessment will use. BS 4142 works to reference time intervals of one hour during the day and 15 minutes at night, with the day running 07:00 to 23:00 and the night 23:00 to 07:00. A residential scheme assessed against BS 8233:2014 internal levels needs enough of both to characterise a typical weekday and a weekend.
Things that force a longer survey:
- A source that operates on a weekly cycle, such as weekend-only deliveries or a Sunday market
- Neighbouring uses with term-time and holiday patterns, including schools
- Seasonal sources such as agricultural drying plant or outdoor hospitality
- A background that is dominated by road traffic which drops sharply at night
Avoid school holidays and public holidays unless the assessment is about those periods. Also avoid a week when the adjacent road is closed for resurfacing, which sounds obvious until it happens.
Free field and facade positions
Position is where surveys most often go wrong, because the choice changes the number.
| Position | Typical use | What to record |
|---|---|---|
| Free field, 1.2 to 1.5 m above ground, away from reflecting surfaces | Background sound level for a BS 4142 assessment | Distance to nearest reflecting surface and to the boundary |
| 1 m from a facade | Incident level at an existing or proposed window | Storey, window served, and that the facade correction applies |
| At the notional receptor position | Where the assessment location is a proposed building not yet built | Justification for the chosen location and height |
A measurement taken 1 m from a hard facade includes reflected sound and reads around 3 dB higher than the equivalent free field value. State which convention each figure follows. A report that mixes the two, then compares them with a criterion expressed in the other convention, produces an error larger than most mitigation will fix.
Height matters as much as plan position. On a scheme with upper-floor bedrooms facing a road, a ground level measurement taken behind a wall tells you very little about the fourth floor. Where access prevents measurement at the right height, say so and explain how you have accounted for it.
Calibration and drift checks
Use a Class 1 instrument. Field calibrate before the logger is left and again when it is collected, record both readings, and report the drift. A drift of more than about 0.5 dB puts the dataset in question, and the honest response is to say what you found rather than to present a mean.
Keep the laboratory calibration certificates for both the meter and the acoustic calibrator in the appendix, with their dates. A certificate that expired midway through the survey is the sort of detail a consultee spots quickly.
Fit the windshield, check the battery and memory capacity against the intended duration, and log the meter's clock against a reference time. Time errors of a few minutes matter when you are correlating events with a weather record.
Weather screening and data exclusion
Rain on a microphone produces spurious high levels. Wind produces both self-noise and genuine changes in propagation.
Record weather data for the survey period from an on-site logger where the site justifies it, or from the nearest representative station, and note the distance to it. Then apply exclusions consistently: periods of rainfall, and periods where wind speed exceeded the threshold you have set, commonly 5 m/s. Downwind and upwind conditions change propagation from a fixed source, so note wind direction relative to the source and receptor as well.
The discipline that matters is transparency. Do not quietly delete the awkward hours. Tabulate what was excluded, for how long and why, and show the remaining coverage per period. An assessment based on 17 valid night-time periods out of 21 is perfectly usable when it says so. The same assessment presented as 21 clean nights is not.
Logging indices and why they matter
Set the meter to log the indices your assessment needs, at the resolution you will want later, and accept a larger file. Going back for a second survey because you logged only 15-minute LAeq values is an expensive lesson.
For most planning work that means LAeq,T for the ambient and specific sound, LA90,T for the background, LAFmax for individual night-time events, and LA10,T where a road traffic method is involved. Log at short intervals, typically one minute or five minutes, so that periods can be rebuilt after exclusions without re-running the survey. Store the third-octave data if the source might need a tonality correction under BS 4142.
Where servicing and deliveries drive the assessment, the event statistics matter more than the average, and our note on night deliveries and servicing noise covers how those events get treated.
Presenting raw data in an appendix
A reviewer should be able to rebuild your headline figures from the appendix without asking for anything. That means a time history chart per position, tabulated period statistics, the weather record aligned to the same clock, the exclusion log, calibration records, a plan showing measurement positions with dimensions, and photographs of each position.
Photographs do real work here. They show that the free field position was actually free field, and they settle arguments about reflecting surfaces without a site visit.
As an illustrative example, a five-night unattended survey at a proposed flat above a parade of shops might yield a night-time background of 38 dB LA90,15min on three nights, 46 dB on a fourth when a nearby road was closed and diverted traffic past the site, and an excluded fifth night after heavy rain. The defensible report uses the three representative nights, explains the fourth, and shows the fifth as excluded with the rainfall record beside it.
Councils usually expect a noise assessment to be written by a suitably qualified acoustician, and professional review is strongly advised before submission. PlanScope can draft the structure and policy framing of a noise assessment, flag where site measurements are still needed, and then route the draft for professional review. Enter your postcode on PlanScope to see which reports a site is likely to need, and read our note on what an acoustic barrier actually achieves before assuming mitigation will close a gap. Once permission is granted, measured levels can still matter, as set out in our piece on statutory nuisance after permission.
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. For specialist technical reports, qualified professional input may be required.
Frequently asked questions
How long should an unattended noise survey run?
Long enough to represent the conditions the assessment relies on, rather than a fixed number of days. Cover typical weekdays and a weekend, extend where the source or the neighbouring uses run on a weekly or seasonal cycle, and avoid school and public holidays unless those periods are the point. Record why the chosen period is representative.
What is the difference between free field and facade measurements?
A free field position sits away from reflecting surfaces, usually 1.2 to 1.5 metres above ground. A facade position sits about 1 metre from the building and includes reflected sound, reading roughly 3 dB higher. State which convention each figure uses, because comparing one against a criterion written for the other introduces an error larger than most mitigation.
Which noise indices should the logger record?
For most planning work, LAeq for ambient and specific sound, LA90 for background, LAFmax for night-time events and LA10 where a road traffic method applies. Log at one or five minute intervals so periods can be rebuilt after exclusions, and keep third-octave data where a tonality correction under BS 4142 may be needed.
Can I delete data affected by rain or wind?
Exclude it, but never silently. Set the thresholds in advance, commonly any rainfall and wind speeds above 5 m/s, then tabulate what was excluded, for how long and why, and show the valid coverage that remains. A survey resting on 17 of 21 night-time periods is usable when it says so plainly.
Does an unattended survey remove the need for an attended visit?
No. A logger establishes the statistical baseline but cannot identify sources or judge character. The tonality, impulsivity and intermittency corrections in BS 4142 depend on an engineer listening on site. Most defensible assessments combine several unattended days with short attended periods covering the operating conditions that matter.
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.