This is the single most important concept in noise measurement, and also the one most often left out of reports. If you read only one article about measurements, read this one.
The problem it solves
A meter placed by a residential building picks up everything: traffic, wind in trees, neighbouring businesses, aircraft, people — and the installation that is the subject of the complaint.
If the result is 47 dB(A), that number alone cannot tell you how much of it comes from that installation. Possibly almost all of it. Possibly almost none.
The solution
The residual noise measurement
Measure twice: once with the installation running, once with it switched off, in conditions as similar as possible. The second result is the residual noise — everything else in the background without the source under investigation.
The difference between the two shows how much that installation adds.
How to read the difference
Decibels are a logarithmic scale, so the meaning of a difference is not intuitive.
| Difference | What it means |
|---|---|
| 10 dB or more | The source dominates completely; background is irrelevant |
| 6–9 dB | The source is the main cause of the noise |
| 3 dB | Source and background contribute roughly equally |
| 1–2 dB | The source is detectable, but background dominates |
| Under 1 dB | The source's contribution cannot be reliably determined |
Important: even a small difference matters at night. Where the background is low and steady, a constant addition of 1–2 decibels can be precisely the sound that prevents sleep — because it is the only one that never stops.
What ruins the comparison
For the two measurements to be comparable they must be obtained on the same basis. In practice four things break that:
- Different data processing. If passing-vehicle events were removed from one series but not the other, the two are not directly comparable — one has been cleaned, the other has not.
- Different weather. If one measurement was taken with wind from the source and the other with a headwind, the difference reflects the wind, not the installation.
- Different time of night. A measurement at 23:00 and one at 03:00 are entirely different background conditions.
- A missing period. If residual noise was measured only at night, nothing can be said about the evening.
What to check in a report
Open the report and check four things:
- Was a residual noise measurement taken at all?
- Was it taken in every period — day, evening and night?
- Are the same weather conditions recorded for both series?
- Was the same processing applied to both, or does one carry separate notes about removed events?
If the answer to the second is "no", that is a fair question to put to the competent authority — not an assertion that the measurement is wrong, but a question about whether any conclusion can be drawn for the missing period.
Why this also serves the company
A residual noise measurement is not only in residents' interests. If it turns out the installation's contribution is minimal, that is the strongest argument a company can have — far stronger than a bare "the limits are not exceeded".
So when a company commissions measurements, including residual noise in the brief serves both sides. It is one of the few things in cases like this where the interests actually coincide.
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.
