Choosing the measurement position for a BS 4142 survey
Two acousticians can reach opposite conclusions on the same plant without an arithmetic error. The measurement position is usually the reason, and it has to be justified in writing.
Two acousticians can assess the same plant installation against BS 4142 and reach opposite conclusions without either making an arithmetic error. The difference is usually the measurement position. Move the microphone from the garden boundary to the façade of the nearest bedroom, or from a shielded courtyard to an exposed elevation, and the rating level and the background sound level both shift, often in opposite directions.
BS 4142:2014+A1:2019 says less about position than practitioners would like, which is why the justification has to be written down rather than assumed.
Why position changes the answer twice over
The standard compares a rating level, derived from the specific sound source, with a background sound level expressed as the LA90 over the reference time interval. The excess of one over the other drives the initial estimate of impact.
Position affects both terms, and not by the same amount. Moving closer to the source raises the specific sound level. Moving away from a busy road lowers the background. A position that is close to the plant and screened from the road traffic therefore produces the worst possible outcome for the applicant, and a position in an open garden near a road produces the most favourable one.
Neither is wrong in itself. What matters is whether the position stands for the assessment location, meaning the place where the effect on a person actually occurs.
Free field and façade positions
A free field measurement is taken away from reflecting surfaces, conventionally at least 3.5 metres from any reflecting surface other than the ground. A façade measurement is taken close to a building elevation, typically within a metre or so, and includes the reflected energy from that surface.
The reflection adds roughly 3 dB. Where measurements are made close to a façade, subtracting 3 dB gives the approximate free field equivalent, and the standard is explicit that this correction should be applied when converting.
The error that appears most often in review is inconsistency rather than the choice itself. A specific sound level measured at the façade compared against a background measured in the free field carries an unstated 3 dB bias, and it survives peer review more often than it should. State for every measurement whether it is free field or façade, and apply the correction once.
| Position | Typical use | Correction |
|---|---|---|
| Free field, 1.2 to 1.5 m above ground | Gardens, amenity space, open ground | None |
| Façade, within about 1 m of the elevation | Where free field access is impossible | Subtract about 3 dB for free field equivalence |
| Upper floor window | Bedrooms above ground level, plant at roof level | Note height and access method |
Choosing the assessment location
Start from the receptor rather than the instrument. Ask where the effect arises: a bedroom window at night, a living room window in the evening, a garden on a Sunday afternoon, a school playground during the day.
For a night-time plant assessment the assessment location is normally the nearest noise-sensitive façade at the floor level of the bedrooms. For a daytime amenity assessment it may be the garden. The two can give different answers because screening, distance and height all change between them, and a full assessment often needs both.
Where the nearest receptor is not the most exposed receptor, say so and assess both. A terrace at 40 metres with an intervening building can be less exposed than a dwelling at 80 metres with a clear line of sight, and an assessment that only reports the nearest property is incomplete. What an intervening structure actually delivers is a separate question, and what an acoustic barrier achieves in practice is usually less than the drawing suggests.
Measuring at a receptor that does not exist yet
Assessing a proposed development against existing industrial or commercial sound means the receptor is a future façade on a drawing.
The workable approach is to measure at the position the façade will occupy, in the free field, and record the ground level and the proposed finished floor levels so that the height relationship is traceable. Where the ground is inaccessible or unsafe, measure at the nearest practicable position and correct by calculation, showing the propagation model, the distances and the assumptions.
Two things need stating explicitly in that situation. Whether the measured background will change once the development is built, since a new building alters screening and reflection for the whole area. And whether the assessment assumes windows open or closed, because that assumption connects the external position to the internal criteria and it is the assumption that conditions are later written against.
Height deserves the same treatment. Plant at roof level on an adjacent building may be inaudible at ground level and clearly audible at second floor, and a ground level measurement in that situation understates the effect at the receptor that matters.
Recording the position so somebody can repeat it
A measurement nobody can reproduce is a measurement nobody has to accept. The record for each position should allow a reviewer to stand in the same place with the same equipment.
- A photograph looking towards the source and a second looking away from it
- Grid reference or coordinates, and the position marked on a scaled site plan
- Microphone height above local ground level, stated as a figure
- Distance to the nearest reflecting surfaces, with the free field or façade designation
- The line of sight to the source, and any screening between them
- Weather throughout the measurement period, including wind speed and direction relative to the source, temperature and rainfall
- Calibration before and after, with the drift recorded
- An audio log or observer notes identifying extraneous sources
Wind direction is the item most often omitted and the one that most often undermines a night-time dataset. Downwind measurement raises received levels, upwind suppresses them, and a background survey taken in a single wind direction is not representative of the year.
Where position gets disputed
Objections from environmental health officers and at appeal follow recognisable patterns, and most are avoidable at survey stage.
The recurring criticisms are that the position was shielded from the specific source by a structure that will be removed, that it was dominated by an extraneous source such as a road so that the background is overstated, that it was on the wrong floor for the sensitive rooms, that the specific and background measurements were taken at different positions or different heights, and that the position was chosen for convenience of access rather than representativeness.
The defence is written before the criticism arrives. Set out in the methodology why the position was selected, which alternatives were considered and rejected, and what the effect on the result would have been had a different position been used. A short sensitivity paragraph showing the outcome at the alternative position is more persuasive than an assertion that the chosen one was appropriate.
Councils normally expect a suitably qualified author for acoustic work, and professional review of any draft is strongly advised before it is submitted as part of a Noise Impact Assessment. Where the source is servicing rather than fixed plant, the position question interacts with timing, and assessing night deliveries and servicing needs the measurement periods to match the activity rather than the reference interval alone.
One last point that outlives the application. The planning assessment and any later statutory nuisance investigation are separate regimes with different tests, and what happens when nuisance is alleged after permission is granted frequently turns on whether the original survey positions bore any relationship to where people were actually affected.
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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
Should BS 4142 measurements be free field or at the façade?
Either can be appropriate, but they must be consistent. A free field position sits at least 3.5 metres from reflecting surfaces other than the ground. A façade position includes roughly 3 dB of reflected energy, so subtract about 3 dB to obtain the free field equivalent. Comparing a façade specific level with a free field background introduces an unstated bias.
What height should the microphone be at?
For ground level free field positions, 1.2 to 1.5 metres above local ground is conventional. Where the sensitive rooms are on upper floors and the source is at roof level, a ground measurement can understate the effect, so assess at the relevant floor level or correct by calculation and show the propagation assumptions.
How do you measure at a receptor that has not been built?
Measure in the free field at the position the future façade will occupy, and record existing ground level alongside the proposed finished floor levels so the height relationship is traceable. Where access is not possible, measure at the nearest practicable point and correct by calculation, showing distances and the propagation model used.
Which reference time intervals does BS 4142 use?
One hour during the day and 15 minutes at night. The background sound level is the LA90 over that interval, and the rating level is the specific sound level with any character corrections applied. Choosing intervals that capture the source operating typically, rather than at its quietest, is part of representative measurement.
Why does wind direction matter so much?
Downwind propagation enhances received levels and upwind suppresses them, so a background survey taken entirely in one wind direction is not representative. Record wind speed and direction relative to the source throughout, along with temperature and rainfall, and exclude periods where conditions fell outside the range you have stated.
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.