Choosing the week for a baseline noise survey

Pick a week in which the background sound level is representative of normal conditions at the receptor, and avoid the five or six weeks each year when it is not. That is the whole of noise survey timing in one sentence, and getting it wrong is the most common reason an environmental health officer asks for a repeat survey.

The background level matters more than the level of your own plant, because BS 4142:2014+A1:2019 assesses the difference between the two. Measure during a quiet school holiday week and your plant looks worse than it is. Measure during a week with a diversion routing lorries past the facade and it looks better than it is, which is the version the officer will reject.

What the survey has to represent

BS 4142 deals with sound of an industrial and commercial nature, which in practice means fixed plant: kitchen extract, air conditioning condensers, cellar cooling, refrigeration, air source heat pumps, compressors and generators. It is not the right method for music, for road traffic or for children at play.

The method needs three quantities. The specific sound level, which is the level of the source being assessed at the receptor. The residual sound level, which is everything else. And the background sound level, expressed as the LA90 over the relevant period, meaning the level exceeded for 90 per cent of the time.

The rating level is the specific sound level plus corrections for acoustic character, with tonality, impulsivity, intermittency and other distinguishing features each attracting a penalty. The assessment then compares the rating level with the background level and considers the context. A difference of around plus 10 dB or more is an indication of a significant adverse effect, a difference of around plus 5 dB is an indication of an adverse effect, and the lower the rating level relative to background the less of an issue the sound is likely to be.

The reference time intervals that set the measurement

Two periods matter and they are not the same length.

So the night-time assessment uses a fifteen minute reference interval, which makes it sensitive to short quiet spells. On a suburban street the lowest fifteen minute LA90 of the night typically falls somewhere between 02:00 and 04:00, and that is the figure your plant has to be compared against if it runs continuously.

Decide first whether the plant runs at night. A kitchen extract that shuts at 23:30 needs a daytime assessment plus a short evening consideration. A cellar cooler that cycles all night needs the full night-time dataset, and that drives the survey length.

Weeks and days that distort the background

Four categories of week to avoid, and they cover more of the calendar than people expect.

• School holidays, particularly the summer break and the weeks either side of Christmas and Easter, when road traffic on local roads drops and the background with it.
• Bank holiday weekends and the working days adjacent to them.
• Weeks with local events, roadworks, diversions or a major closure on a nearby route, either of which shifts the traffic contribution in one direction or the other.
• Weeks when a neighbouring industrial or commercial operation is shut for maintenance or seasonal closure, which removes part of the residual sound the receptor normally experiences.

Day of the week matters as well. A Sunday night in a residential area will usually give the lowest background of the week. Where the plant operates seven days, that is the condition to measure. Where it operates Monday to Saturday, a midweek night is the representative one, and measuring on the quietest night of the week overstates the case against your own scheme.

Season has a smaller effect than people assume, with one exception. Summer is the condition in which windows are most likely to be open, so an assessment of effect inside a dwelling should consider a partially open window rather than relying on a closed, sealed facade. Deciduous vegetation provides almost no reliable attenuation and should not be claimed.

Weather limits and how to record them

Measurements taken in rain or in strong wind are unusable, and an officer who sees no weather record will assume the worst. Standard practice is to exclude periods of rainfall and periods when wind speed at the microphone exceeds about 5 m/s, to fit an appropriate windshield throughout, and to note wind direction because a downwind receptor sees higher levels from the source than an upwind one.

Record the weather from a source you can cite, either an on-site logger or the nearest published station with its distance stated. Then show the exclusions on the time history graph rather than silently deleting them. A dataset with four hours removed and a reason given is credible. A dataset with four hours missing and no explanation is not.

Attended samples alongside unattended logging

Unattended logging over five to seven days gives the statistical picture. Attended measurement gives the explanation, and a report with only the first is weaker than it looks.

Spend two or three periods on site with the meter, one in the daytime and at least one at night, and write down what you hear. Which sources are audible, whether the plant next door is tonal, whether there is a distant road hum, whether a railway passes. Those notes are what justify the character corrections later, and they are what the environmental health officer compares against their own experience of the street.

Position the microphone where the assessment needs it, normally at the nearest noise-sensitive facade at the relevant floor level, with a free-field or facade measurement clearly identified. A measurement taken at ground level in a rear garden cannot be used for a first floor bedroom window without a stated correction.

Agreeing the period with environmental health

Most authorities will discuss survey scope before the work is done, and the conversation takes twenty minutes. Put four things in writing.

1. The monitoring positions, with a plan.
2. The survey duration and the dates proposed, with the reason the week is representative.
3. The standard and version being applied, and any supplementary guidance the authority expects.
4. Whether a planning condition will require a post-completion verification measurement, because that affects how the design margin is set.

Where a condition is already in place requiring noise evidence before works start, the survey window has to fit the construction programme, and the sequencing issues in discharging pre-start conditions for a spring start apply directly. Where the survey will straddle a year end, the budgeting problem is covered in planning a survey budget across two financial years.

What the report must disclose

A Noise Impact Assessment that an officer can rely on states all of this on the face of the document: instrument type and class, calibration before and after with the drift recorded, microphone height and position, survey dates and times, weather conditions and exclusions, the reference time intervals used, the measured background levels for each period, the character corrections applied and why, and the context discussion.

Two omissions recur. No stated reason why the survey week was representative, which is the gap this article exists to close. And no acknowledgement of uncertainty, where a single quiet night has been used as the design case without saying so.

Illustrative example: a takeaway proposing a new kitchen extract fan behind a terrace, with the fan running until 23:00. A five day unattended survey in late September, avoiding the half term week, with attended samples on a Wednesday evening and a Thursday night, gives a daytime and evening background against which the fan can be specified, and the extract route and discharge height then go into the Ventilation/Extraction Statement alongside the odour case.

Browse the 66 report types to see which environmental documents your proposal is likely to need, and have the acoustic work prepared or reviewed by a suitably qualified acoustician. This is general guidance, not legal advice, so 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.