SOAEL and LOAEL: using the noise effect levels properly

SOAEL LOAEL planning work goes wrong at the first step, when an assessor treats the two abbreviations as numbers waiting to be looked up. They are not. The Noise Policy Statement for England sets out a hierarchy of effect levels, the lowest observed adverse effect level and the significant observed adverse effect level among them, and it deliberately declines to attach national figures to either. Planning Practice Guidance carries the same hierarchy into decision taking with the same silence on numbers.

That silence is the whole design of the policy, and an assessment that works with it produces a far stronger conclusion than one that tries to fill the gap with a borrowed value.

The effect levels and what each one means

The hierarchy runs from no effect through to unacceptable effect, with two named boundaries.

Below the lower threshold, noise is present but produces no adverse effect on health or quality of life. At and above it, an adverse effect begins: sleep is disturbed occasionally, windows get closed, people notice. Above the upper threshold, the effect is serious enough that national policy expects it to be avoided, with material changes in behaviour and in quality of life.

The policy's three aims attach to those bands. Avoid effects above the upper threshold. Mitigate and minimise effects between the two. And, where possible, contribute to improving health and quality of life. Those three aims are the sentences a conclusion should echo, because they are what the decision maker is applying.

Why the policy sets no national values

Two people exposed to the same measured level respond differently depending on what produced it, when it happened and what they were doing at the time.

A steady 45 dB from distant road traffic in a suburb reads as background. The same level from a compressor with a tonal whine, starting at two in the morning in a quiet rural village, is a different experience entirely. Character, time of day, existing ambient conditions and the expectations of the area all move the point at which effects begin.

A single national number would therefore be wrong in most places. The policy sets the framework and leaves the values to the assessment, which is more demanding but produces better answers.

How values are established in practice

Authorities and assessors converge on a workable method even without national figures.

The usual approach draws on established acoustic standards for the receptor type, applies them to the relevant time periods, and adjusts for the character of the source and the existing conditions on the site. Internal guideline values for resting and sleeping inform the design target. External amenity guidance informs the garden assessment. Where the source has an industrial or commercial character, comparison against the measured background sound level gives an indication of impact in its own framework.

The step that separates a good report from a weak one is what happens next. The evidence from those standards has to be converted into a stated position on where the thresholds sit for this receptor, in this location, at this time of day, with the reasoning shown. A table helps.

Fill that in for each receptor and the conclusion writes itself. Leave it blank and the environmental health officer has nothing to agree or disagree with.

Applying the hierarchy to a proposal

Work in the order the policy does.

First establish the existing acoustic environment with measurement, over a period long enough to capture the relevant conditions. Then predict the level at each receptor with the development in place. Then place that level in the hierarchy using the values you have derived and justified.

Where the result sits between the two thresholds, the policy expects mitigation and minimisation. That is a design conversation about layout, orientation, glazing and ventilation strategy, screening and operational control. Be realistic about what screening delivers, because what an acoustic barrier achieves in practice is often less than the specification implies once diffraction and flanking are taken into account.

Where the result sits above the upper threshold, say so plainly and set out the response. An assessment that quietly adjusts its threshold upward until the scheme complies is the single most common reason a consultee objects, and it is easy to spot.

Mechanical plant deserves separate treatment, because it runs when everything else is quiet. Kitchen extract in particular combines noise with odour and with an elevation problem, and the Ventilation and Extraction Statement should be consistent with what the acoustic report assumes about the plant specification.

Where the thresholds get argued

Appeal decisions engage with these terms regularly, and two patterns recur.

Inspectors have been unimpressed by assessments that assert a threshold without derivation. They have also been unimpressed by objections that assert a lower threshold with equally little support. Reasoned values, applied consistently, tend to survive. Bare assertion on either side does not.

The other recurring point concerns whether an effect that can be mitigated by closing windows counts as mitigated. Treat that carefully. A design relying on sealed windows has to deal with overheating and with ventilation, and a conclusion that depends on occupants behaving in a particular way is weaker than one that does not. Appeal decisions are indicative of how inspectors have approached the issue rather than binding precedent.

Operational noise after permission has its own regime, and the relationship between a granted permission and statutory nuisance after permission catches operators out. Permission is not a defence to nuisance in most circumstances.

A related question is whose baseline you are measuring. On a site where an existing use is about to cease, the background sound level measured today may disappear along with it, and an assessment resting on that background will overstate what the new receptors can tolerate. Measure the condition that will exist, explain the assumption, and if the existing source is staying, say what secures it.

Common misuse in submissions

Four errors account for most of the objections.

Using an internal guideline figure as though it were the policy threshold. Reporting a standard's own impact conclusion as though it were a finding under the policy hierarchy. Applying a daytime value to a night-time period. And averaging across a period long enough to hide the events that cause the effect, which is the problem with night-time servicing noise assessed on a sixteen hour metric.

None of these is hard to avoid. All of them are easier to avoid before the report is written than after a consultee has read it.

Councils usually expect a suitably qualified author for acoustic work, and a Noise Impact Assessment should be reviewed by one before submission. For specialist technical reports, qualified professional input may be required. This is general guidance rather than 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.