Measurements · Noise
Statistical Level (Ln)
Also known as percentile noise level, L90, L10
Statistical / percentile level (e.g. L90 background, L10 intrusive).
What it is
What are statistical levels (Ln)?
Statistical, or percentile, levels describe how noise is distributed over time rather than reducing it to a single average. The notation Ln means the level exceeded for n percent of the measurement period. L10 is the level exceeded for 10 percent of the time (the higher, intrusive level dominated by louder events), while L90 is the level exceeded for 90 percent of the time, taken as the residual or background level. L50 is the median. They are normally A-weighted and quoted over a stated period, such as LA90 over an hour. Together a set of Ln values paints a far richer picture than a single number, showing both the busy peaks and the quiet floor of a noise climate.
Where Leq compresses the whole distribution into one energy-average level, the statistical levels open it back up, which is why they are the workhorse of community and planning noise assessment.
Health and operational effects
Community noise disputes turn on how a new or existing source relates to the existing environment, and the statistical levels are what make that comparison possible. L90 defines the background against which an industrial or commercial source is judged, because a new sound well above the residual level is far more likely to be noticed and to provoke complaints. The difference between a source's level and the background is central to industrial noise assessment methods. L10 has long been used to characterize road-traffic noise, capturing the intrusive upper end that residents notice.
For planners, operators, and consultants, the statistical levels are therefore the language of nuisance and mitigation: the numbers that determine whether a proposal is acceptable and what controls it needs.
How it is measured
How Statistical Level (Ln) Is Measured
Ln values are computed by a sound level meter built to the IEC 61672 standard, which samples the level many times a second and, at the end of the period, reports the value exceeded for each chosen percentage of the time. Meaningful statistics require a representative measurement period and clean data, free of wind and other artifacts that would distort the distribution. Because these are full acoustic metrics beyond a simple ambient reading, they are measured with an external IEC 61672 instrument logged on a Thiamis gateway, reported alongside the anchor metric Leq so that both the average and the distribution are available.
An instrument on Thiamis1
A third-party instrument connected to a Thiamis gateway, reporting into the same Environet account.
Placing noise monitors
For community and background monitoring the microphone is placed at the affected receptor or the site boundary, at standard height, clear of reflecting surfaces, and fitted with a windshield, since wind corrupts the low end of the distribution and inflates L90. Continuous logging over a representative period (often including quiet nighttime hours, when the background is lowest and most relevant) is essential because the statistical levels only mean something over a properly sampled span. Consistent, standard-compliant positioning is what makes the resulting L90 and L10 defensible.
Every device and option that measures Statistical Level (Ln) 1
| Device / option | Type | Base platform | Measures |
|---|---|---|---|
 Cirrus Research · 151B, 152B, 151A… | Noise Monitors | Thiamis |
Where it is monitored
Solutions That Monitor Statistical Level (Ln)
Each one links to the solution page showing what that industry measures it for.
Noise Monitoring
Core parameterStatistical noise levels (Ln) describe how noise is distributed over time: L90 characterizes the background and L10 the intrusive peaks.
Perimeter Monitoring
Often addedSite boundary dust, silica, and noise compliance
Related
More in Noise
Questions
Statistical Level (Ln) FAQ
What do L10 and L90 mean?
L10 is the level exceeded for 10 percent of the time, representing the louder, intrusive end of the noise, and L90 is the level exceeded for 90 percent of the time, taken as the residual background. Reading them together shows both the peaks people notice and the floor the environment sits at.
How are statistical levels different from Leq?
Leq is a single energy-average level for the whole period, whereas the Ln values describe the distribution of levels over that period. Leq tells you the overall exposure; the statistical levels tell you how variable it was and what the background and intrusive levels are.
Why does background level matter so much?
A new source is judged largely by how far it rises above the existing background, and L90 is the accepted, robust measure of that background. The greater a source's margin above the residual level, the more likely it is to be noticed and to attract complaints.
How do I monitor statistical noise levels?
Statistical metrics such as L10 and L90 need an external IEC 61672 sound level meter on a Thiamis gateway, logging continuously over a representative period alongside Leq. Use the configurator to combine noise with the other parameters your community or planning assessment requires.
Monitor Statistical Level (Ln) on the Record
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