Background sound level and why BS 4142 depends on it

The background sound level in a BS 4142 assessment is the LA90 measured at the assessment position with the specific sound switched off, taken over a period that represents the times the source will actually run. Every conclusion in the method sits relative to that number. BS 4142:2014+A1:2019 does not ask whether a chiller or an extract fan is loud in absolute terms. It asks how far the rating level of the specific sound sits above the background sound level, and it reads that margin as an indication of likely impact. An assessment with a careless background figure has no anchor at all.

Which is why reviewers read the background section first, before the source data, before the modelling, before the conclusion.

What the background sound level measures

The background sound level is the residual sound of the place without the thing being assessed. Road traffic a street away, distant rail, wind in trees, the hum of a town at three in the morning. It is measured at the assessment position, which usually means at or near the facade of the nearest noise-sensitive window, or in the garden where amenity is the concern, and at a height that reflects where people are affected.

Two details are often fudged. The source must be off or absent during the measurement, which is easy on a new installation and hard on a retrospective assessment of plant that cannot be switched off. And the microphone has to be where the receptor is, not somewhere more convenient at the site boundary where the background happens to be higher.

Where the specific sound cannot be turned off, the honest route is to measure at a location with the same acoustic character and no influence from the source, then justify the transfer. Reviewers accept that if the reasoning is written down. They do not accept a background figure quietly borrowed from a different site.

Why the LA90 statistic is used

LA90 is the A-weighted sound pressure level exceeded for 90 per cent of the measurement period. It strips out the short, loud events that dominate an LAeq, so a passing motorbike, a car door or a dog does not inflate the figure. What remains approximates the steady underlying level a listener perceives as the ambient condition.

That matters because the method compares a continuous or quasi-continuous plant sound against the level people are accustomed to. If you used LAeq as the baseline, a handful of transient events would raise the baseline, shrink the apparent excess and make almost any installation look acceptable. The LA90 is deliberately conservative in the receptor's favour.

It also means a quiet rural site produces a low number with nothing to hide behind.

Choosing a representative period

Representative means representative of the source's operating hours, not representative of a convenient afternoon. If the plant runs continuously, the daytime period (07:00 to 23:00) and the night period (23:00 to 07:00) both need their own background figures, because the standard applies a different reference time interval to each: one hour for daytime and 15 minutes at night.

Four questions decide whether a period is defensible:

1. Does the measurement cover the hours the source will run, including the quietest of them?
2. Is the weather within the limits the equipment and the standard tolerate, with wind speed and rainfall logged?
3. Are the days typical, avoiding school holidays, local events, roadworks and anything else that changes traffic flow?
4. Is there enough data to show the pattern rather than a single snapshot?

Unattended monitoring over several days and nights, with attended spot measurements to identify what is audible and when, is the arrangement most environmental health officers expect for anything beyond a single small unit. A short attended survey can be enough for a domestic air source heat pump. It is rarely enough for a commercial kitchen extract above a flat.

Typical against lowest measured values

Here is where assessments diverge most. Over a week of night-time monitoring you will have dozens of 15-minute LA90 values. Which one is the background sound level?

BS 4142 asks for a value representative of the period, and leaves the practitioner to derive it and justify the choice. In practice three approaches appear:

An acoustician acting for an applicant will reach for the typical value. An officer worried about complaints will ask what happens at the lowest. The sensible answer is to present both, assess against the typical figure as the primary case, and show the margin at the lowest value as a sensitivity test. That takes one extra table and removes the main line of attack.

Illustrative example. On a suburban site, night-time 15-minute LA90 values cluster around 33 dB with a quietest interval of 28 dB. A new condenser is predicted to give a rating level of 38 dB at the nearest bedroom window. Against the typical background the excess is 5 dB. Against the quietest interval it is 10 dB. The first points towards mitigation being advisable, the second towards it being necessary, and the honest assessment says so and specifies an attenuator rather than choosing the flattering number. The figures here are illustrative only.

The rating level excess and its bands

The rating level is the specific sound level at the receptor plus any character corrections for tonality, impulsivity, intermittency and other distinctive features. Subtract the background sound level and you have the excess that drives the conclusion.

BS 4142 gives indicative guidance rather than limits. A difference of around +10 dB or more is an indication of a significant adverse impact, depending on context. Around +5 dB is an indication of an adverse impact, again context-dependent. The lower the rating level relative to the background, the less likely an adverse effect, and a rating level at or below the background is an indication of low impact.

Context is doing real work in those sentences. The standard expects the assessor to consider the absolute level, the character of the area, the sensitivity of the receptor and the acoustic design of the receiving building. A +6 dB excess next to an existing industrial estate reads differently from +6 dB in a cul-de-sac. That judgement belongs to a qualified acoustician, and it is the part of a Noise Impact Assessment a reviewer should be least willing to rubber-stamp.

Councils often convert the outcome into a condition expressed as a rating level not exceeding the background by a stated margin at a stated position. A condition drafted that way has to satisfy the six tests a planning condition must meet, and it has to be measurable after the event, which is another reason the background derivation needs to be reproducible.

Low background sites and why they are hard

Rural and semi-rural sites routinely produce night-time LA90 values in the high twenties or low thirties. Below about 30 dB the practical problems stack up. Instrument noise floors become relevant. Insect and bird sound in summer can lift the LA90 above the true residual. And a 10 dB excess over a 28 dB background is a rating level of 38 dB, which is quiet in absolute terms yet still an indication of adverse impact under the method.

This is the situation where applicants argue that the absolute level is low enough not to matter and officers argue that the method is the method. Both have a point. The useful response is to run the assessment properly, state the absolute rating level alongside the excess, and reference the internal levels the receptor would experience with windows open and closed against BS 8233 guideline values. Mitigation on a low background site usually means acoustic enclosures, low-speed fans, siting away from the boundary and a night-time setback, agreed before submission rather than argued after refusal.

Documenting the derivation so a reviewer can follow it

A background section a reviewer can sign off contains the measurement positions with a plan and photographs, the equipment make, model and calibration record, the dates and times, the weather data, the full time history of LA90 values, the derived background figure for each period, and the reasoning that connects the data to that figure.

Planning Practice Guidance on noise expects the effect of noise to be assessed on a case-by-case basis with the evidence set out, and the Noise Policy Statement for England supplies the policy aims that sit behind it. Neither tells you which LA90 to pick. That is a professional judgement, and an assessment that hides it behind a single unexplained number invites a holding objection from environmental health.

PlanScope can draft the structure of a noise chapter and the policy framing around it, with a "to be confirmed" flag wherever site measurements are needed, so the acoustician spends their time on the survey design and the derivation rather than on the narrative. The measured data and the professional judgement have to come from a suitably qualified practitioner, and councils expect that author to be identified. You can see which report types sit alongside a noise chapter on the full list of 66 report types, and reviewers who sign off drafts are listed in the professional directory.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.