Almost every noise measurement report contains a line somewhere: "measurement uncertainty ±3 dB" or similar. Most readers skip it. That is a mistake, because this line sometimes decides whether the verdict is "compliant" or "not compliant".
What it is
Uncertainty is not an error and not an instrument defect. It is an honest statement of how precise the method is overall. For environmental noise it is typically around ±3 decibels, and it comes from several sources combined:
- instrument and calibration accuracy;
- microphone placement and how repeatable it is;
- variation in weather during the measurement;
- variability of the background noise;
- measurement duration and the time window chosen.
An honest report always states it. Its absence would be a much worse sign than its presence.
What it means in practice
Example
Measured 53 dB(A), limit 55 dB(A), uncertainty ±3 dB.
The true value most probably lies between 50 and 56 dB(A).
Part of that range is below the limit; part is above it.
This does not make the measurement worthless. It means that close to a limit value, an unambiguous conclusion cannot be drawn from a single number.
Why reports say it was not taken into account
Reports often note that the assessment is given without taking measurement uncertainties into account. That is not a trick — it is standard wording, and there is a reason for it.
How uncertainty should be handled in a compliance assessment is a policy question, not a measurement question. Several approaches are possible:
| Approach | What it means |
|---|---|
| Not applied | The measured value is compared directly with the limit |
| In the operator's favour | A breach only if the limit is exceeded even after subtracting uncertainty |
| In the environment's favour | A breach if the limit is exceeded once uncertainty is added |
The laboratory does not make that choice — the regulation or the authority does. So the laboratory states the uncertainty and leaves the conclusion to whoever applies the standard.
When it is worth raising
Not always. If 47 dB was measured against a 55 dB limit, uncertainty changes nothing — even with +3 dB the result stays well below. Pressing the point there only weakens your position.
It is a fair question when the result sits within the uncertainty of the limit — say 52–58 dB against a limit of 55. Then it is worth asking the competent authority:
Is it sound to conclude that limit values are observed without accounting for measurement uncertainty, and what approach do the applicable standard and regulation provide for?
The second, more often forgotten aspect
Uncertainty also applies to the difference between two measurements. If an installation's contribution is calculated as the difference between measurements with the source on and off, and that difference is 1–2 dB, the uncertainty is larger than the difference itself.
From which follows a two-sided conclusion that should be stated fairly: a small difference does not prove the source contributes a lot — but equally it does not prove it contributes little. It simply sits at the limit of what the method can resolve, and conclusions in either direction have to be phrased carefully.
What this site documents
This article is published by the Jūrkalnes and Irlavas neighbourhood community. We document the industrial noise produced by the outdoor Nestro filter at SIA Livonia Print, Jūrkalnes iela 15/25, Riga. The installation has run 24 hours a day since 2021, and its hum reaches around 12 streets. The residents’ request to the company is a single one: fit an acoustic enclosure around the filter and sound insulation on the air ducts.
