Real-world noise is never steady. Traffic swells and fades, a neighbour's music thumps then stops, a dog barks twice. A single "dB" figure can't describe that, so acousticians use a handful of statistics. You'll find them in planning conditions, licensing reports, workplace assessments and noise complaint letters, and in the Pro mode of a good meter app.
How to read the labels
Most labels are built from the same pieces:
- L means "level", in decibels.
- The next letter is the frequency weighting: A, C or Z. See dBA vs dBC vs dBZ.
- Then sometimes a time weighting: F (Fast), S (Slow) or I (Impulse).
- Then what kind of statistic it is: eq, max, min, peak or a percentage.
- And sometimes a time period after a comma, like "LAeq,1h" for one hour or "LAeq,16h" for the daytime 07:00 to 23:00.
Leq: the average that counts
Leq (equivalent continuous level), usually written LAeq when A-weighted, is the steady level that would carry the same total sound energy as the fluctuating noise over the measurement period.
It's not a simple average of the readings. Because decibels are logarithmic, loud moments dominate. Ten minutes at 50 dB followed by ten minutes at 80 dB gives an Leq of about 77 dB, not 65 dB, because the loud ten minutes carries a thousand times more energy.
Leq is the figure behind almost every noise guideline, from WHO environmental limits to workplace exposure. When someone asks "how loud was it?", Leq is usually the honest answer.
LAmax and LCpeak: the loudest moments
- LAmax (or LAFmax) is the highest A-weighted level reached during the measurement, using Fast or Slow time weighting. Night-time guidance often uses LAmax, because individual loud events (a door slam, a motorbike) wake people even when the average is low.
- LCpeak is the highest instantaneous C-weighted pressure, with no time averaging. It's always higher than LAmax and is what workplace rules use for very short, sharp noises such as hammering or gunshots.
L10, L50, L90: percentiles
These come from sorting all the readings during a measurement.
| Statistic | Meaning | What it tells you |
|---|---|---|
| L10 | The level exceeded 10% of the time | The "noisy" level: regular loud bits like passing traffic |
| L50 | The level exceeded half the time | The median, a typical level |
| L90 | The level exceeded 90% of the time | The background level, the quiet floor between events |
L90 matters in neighbour and planning disputes. A new noise source is often judged by how far it rises above the existing background. In the UK, the standard for assessing industrial and commercial sound (BS 4142) compares the new sound against the background L90. A difference of around 10 dB or more is likely to indicate a significant adverse impact, depending on context.
The gap between L10 and L90 shows how "peaky" a place is. A busy road with a constant roar might have L10 and L90 only a few decibels apart. A quiet street with an occasional car will have a big gap.
SEL: the energy of a single event
SEL (sound exposure level, sometimes LAE) squeezes the entire energy of an event, such as a train passing or an aircraft flying over, into one second. It lets you compare events of different lengths fairly, and add them up: if you know the SEL of one train and how many pass in a night, you can work out the night-time Leq.
Lden and Lnight
Environmental noise maps and the WHO guidelines use Lden (day-evening-night level) and Lnight. Lden is a 24-hour average with 5 dB added to evening noise and 10 dB to night-time noise, because the same sound is more annoying when people are relaxing or sleeping. There's more on how these are used in our guide to road traffic noise at home.
