Most BS 4142 disputes are arguments about the baseline

BS 4142:2014+A1:2019 compares a rating level from a specific sound source against the background sound level at the receptor. The rating level involves character corrections and some judgement. The background level involves standing still with a calibrated meter. It is therefore mildly surprising that in the eleven assessments we were asked to review as part of an appeal last year, the calculations were competent in nine and the background survey was defensible in four.

The failures were not exotic. Three surveys were a single night. Two had no weather record at all, so there was no way to know whether the quietest period had also been the windiest. One had measured the background at a position 60 metres from the nearest actual receptor because access was easier, without saying so. Two had excluded the lowest-measured periods as "anomalous" with no stated criterion, which is the reverse of what the standard intends.

A short survey biases the background low if you catch a quiet night and high if you catch a noisy one, and the direction of the bias determines who it favours. That is the reason it matters: an under-measured background makes a compliant scheme look non-compliant, and an over-measured one gets a genuinely intrusive scheme consented.

What we do, and what we think is the minimum: five consecutive days including a weekend, at the receptor or at a position demonstrably representative of it with the reasoning written down, wind speed and rainfall logged throughout at 10-minute resolution, periods with wind above 5 m/s at 10 metres excluded and the exclusion shown on the timeline rather than silently removed.

It costs perhaps two days more than a single-night survey. On the appeal in question, the inspector’s decision letter turned on the background level, and the survey that survived scrutiny was the one that had logged the weather.

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