Where to place a noise logger and for how long

A noise survey logger position has to answer one question: what does the acoustic environment look like at the point where the assessment matters. That point is a receptor window, a proposed facade, or an outdoor amenity area, and it is rarely the most convenient place to leave a box for a week. Get the position wrong and nothing downstream of it can be relied on, because the background sound level drives the criterion and the criterion drives the mitigation.

Two decisions do most of the work. Where the microphone goes, and how long it stays there.

What the survey has to represent

Start from the assessment method, because the method dictates the measured quantity.

An industrial or commercial sound assessment under BS 4142 needs the background sound level, expressed as the A-weighted sound pressure level exceeded for 90 per cent of the measurement period, over reference periods of one hour during the day and fifteen minutes at night. A residential amenity assessment needs ambient levels to compare against internal guideline values, which means equivalent continuous levels across the day and night periods, plus maximum levels for individual events at night.

Those are different quantities measured over different intervals, and a single logger set to the wrong interval produces data that cannot be reprocessed. Set the logger to store short intervals, commonly one or five minutes, with the full statistical set and the maximum levels, so the data can be aggregated afterwards into whatever period the assessment needs.

Also decide at the outset whether you are characterising the existing environment for a new noise-sensitive development, or establishing a background against which a new source will be judged. The positions differ. For the first, the microphone belongs at the proposed facade. For the second, it belongs at the existing receptor.

Free field and facade positions

Two conventions exist and they are not interchangeable.

A free field position is clear of reflecting surfaces other than the ground, which in practice means at least 3.5m from any wall, fence or large reflecting object. The level measured is the incident sound without a reflected contribution.

A facade position sits close to the building, conventionally within one to two metres of the facade, and the measured level includes the reflection from the wall behind the microphone. That reflection adds approximately 3 dB, so a facade measurement and a free field measurement of the same sound differ by about that amount. Conversions between the two have to be stated rather than applied silently.

Which to use depends on the criterion. Where a standard or a policy expresses its limits as free field values, measure free field or convert and say so. Where the criterion relates to the sound incident on a facade for a glazing specification, a facade measurement is the direct answer.

The commonest error in a submitted survey is a facade measurement compared with a free field criterion, with no correction mentioned. An environmental health officer who spots it will ask for the survey to be repeated.

Height, distance and reflecting surfaces

Height follows the receptor. For a ground floor window, 1.2m to 1.5m above local ground level is conventional. For an assessment affecting upper floors, the microphone needs to be at the height of the most exposed facade, which often means a mast, a scaffold or access to a neighbouring roof.

A logger at 1.5m behind a 1.8m boundary fence is measuring the shadow of the fence rather than the environment at a first floor window. On a site next to a road or a railway, that single error can understate the facade level by a substantial margin and produce a glazing specification that fails on completion.

Other positional checks worth running before you leave site:

• Distance to the nearest reflecting surface, measured and recorded, with a photograph showing the surroundings.
• Ground cover under and around the position, since hard and soft ground give different results for distant sources.
• Proximity to local and atypical sources such as a single air conditioning unit, a gate, a drainpipe or a dog run.
• Security and power, because a survey that stops on day two is not a survey.
• Whether the position is representative of the whole site, and if not, how many positions are needed.

On a constrained urban site two positions, one at the most exposed facade and one in the quietest courtyard, usually tell the story better than one position in a compromise location.

Survey duration and representative periods

There is no single required duration. The requirement is that the measurement period is representative of the conditions being assessed, which means the duration follows the source.

For a background sound level, the value taken forward should be a typical lowest representative level rather than the absolute minimum recorded over the survey. A single quiet quarter hour at 4am on a Sunday is not the background against which a plant installation should be judged, and using it produces a criterion nobody can meet. Say in the report how the representative value was selected.

Where the source is intermittent, attended measurements alongside the unattended logging are worth the time. An hour on site with a hand-held meter and a notebook tells you what the time history plot cannot, which is what each event actually was. The approach to a periodically tested standby generator is a good example, covered in standby generator testing and the noise it creates.

Weather, holidays and atypical weeks

Weather invalidates data. Rain on the microphone, and wind above roughly 5 metres per second, both corrupt readings, so the survey record needs wind speed and direction and rainfall from a nearby station or an on-site sensor, and the affected periods need excluding with the exclusion stated.

Wind direction also matters for a distant source. A week of readings taken with the wind blowing from the receptor towards the source understates the levels, and the report should say which direction prevailed.

Timing is the other trap. Avoid school holidays where traffic is the dominant source, avoid bank holidays, and check for local events, roadworks and temporary diversions. A survey carried out while the adjacent road was closed for resurfacing is worthless, and the only way to know is to ask on site and to look.

Calibration and the audit trail

Calibration is the part an officer can check from the paperwork, so it is the part worth getting right.

Field calibrate with an acoustic calibrator before and after the measurement, record both readings, and state the drift. Where the drift exceeds a small margin, typically half a decibel, the data is suspect and the report should say what was done about it. The instrument and the calibrator both need laboratory calibration traceable to a national standard, within the interval the standards require, and the certificate numbers belong in an appendix.

Also record the instrument type and class, the microphone and windshield used, the frequency weighting and time weighting settings, the storage interval, and the start and stop times. Those details take five minutes to note on site and are impossible to reconstruct later.

Presenting the data so it can be checked

A survey appendix that lets a reviewer follow the logic contains a position plan with distances and heights annotated, photographs from each position in each direction, time history plots covering the whole survey with the excluded periods shaded, tables of the measured levels by reference period, the weather record, the calibration record, and a short explanation of how the representative background was selected.

Put the selection logic in words. Most disputes about a Noise Impact Assessment are disputes about which number was taken forward from the survey, not about the measurement itself. Councils normally expect this work from a suitably qualified acoustician and professional review of any draft is strongly advised.

And keep the raw data. Where a scheme is amended two years later, a reprocessable dataset saves a repeat survey, which matters on mixed use buildings where impact noise and plant noise interact, as in gym impact noise in a building with flats above. Unusual sources with their own temporal pattern, such as those discussed in place of worship noise, bells, calls and gatherings, need the survey designed around the pattern rather than the other way round.

Enter your postcode on PlanScope to see which acoustic and environmental reports your scheme is likely to need.

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.