Measurements · Noise
Min Level (Lmin)
Also known as minimum noise level
Minimum sound level over the interval.
What it is
What is Lmin (minimum sound level)?
Lmin is the lowest time-weighted sound level reached during a measurement period: the quietest moment the meter records as the level fluctuates. Like the other basic metrics it is normally A-weighted and reported with its time weighting, as LAFmin or LASmin. Lmin describes the floor of the noise environment: the residual level that remains when the passing traffic, machinery, and voices briefly fall away. It is closely related to, but not the same as, the statistical background level L90, which is the level exceeded 90 percent of the time and is generally the preferred, more robust measure of background.
Where Lmax marks the loudest instant and Leq the average energy, Lmin marks the quietest instant. Together they bracket the range of the noise climate.
The quietest levels in a period reveal how genuinely calm a location can become, which matters most at night and in rural or protected settings. A low Lmin shows that there are real lulls between events, whereas an Lmin that never drops indicates a continuous underlying source (a distant highway, a plant running around the clock, a ventilation fan) that sets a persistent floor. Establishing this residual level is a key part of a baseline survey, because a new noise source is judged partly by how far it lifts the existing background.
For that reason, Lmin, alongside the statistical L90, helps characterize the existing acoustic environment before a project rather than the impact of any single event.
Band resolution: reported broadband plus 1/1 octave and 1/3 octave. The metric and its frequency band are independent. Check the sound-capable devices below for what each one reports.
How it is measured
How Min Level (Lmin) Is Measured
Lmin is produced by a sound level meter built to the IEC 61672 standard, which tracks the time-weighted level continuously and holds the lowest value reached over the logging interval. Because it belongs to the full set of acoustic metrics rather than a simple ambient reading, Lmin is best measured with an external IEC 61672 instrument logged on a Thiamis gateway, reporting it alongside the anchor metric Leq and the statistical levels that describe the background more completely. A single very brief dip can set Lmin, so it is usually interpreted together with L90 rather than in isolation.
An instrument on Thiamis1
A third-party instrument connected to a Thiamis gateway, reporting into the same Environet account.
Placing noise monitors
For residual and background monitoring the microphone is placed at the affected receptor or a representative point, at standard height, clear of reflecting surfaces, and fitted with a windshield. Continuous logging over days is important because the true quiet periods (the small hours, or gaps between activity) are exactly when the residual level shows itself, and an attended daytime survey would rarely capture them. Siting matters as much as for any noise measurement, since a nearby continuous source will hold the whole distribution, including Lmin, well above the genuine background.
Every device and option that measures Min Level (Lmin) 1
| Device / option | Type | Base platform | Measures |
|---|---|---|---|
 TSI · SP-SE-1, SP-SE-1-1/1, SP-SE-1-1/3… | Noise Monitors | Thiamis |
Where it is monitored
Solutions That Monitor Min Level (Lmin)
Each one links to the solution page showing what that industry measures it for.
Noise Monitoring
Often addedEnvironmental and occupational sound levels
Related
More in Noise
Questions
Min Level (Lmin) FAQ
What is the difference between Lmin and L90?
Lmin is the single lowest time-weighted level recorded, while L90 is the level exceeded 90 percent of the measurement time. L90 is more robust as a measure of background because a brief, untypical quiet moment can pull Lmin down, whereas L90 reflects the level the environment sits at or above most of the time.
Why measure the quietest level at all?
Because the residual, or background, level sets the baseline that a new source is judged against, and it reveals whether a location ever truly falls quiet. A background that never drops points to a continuous source, which is useful diagnostic information in itself.
Does Lmin depend on the time weighting?
Yes. A Fast time constant follows brief dips more closely than Slow, so LAFmin is generally lower than LASmin for the same noise. As with the other metrics, the weighting is always reported with the value.
How do I monitor Lmin and background levels?
Full statistical and residual metrics need an external IEC 61672 sound level meter on a Thiamis gateway, logging Lmin, L90, and Leq continuously over a representative period. Use the configurator to combine noise with the other parameters your survey or permit requires.
Monitor Min Level (Lmin) on the Record
Schedule a callConfigure Min Level (Lmin) Monitoring All parameters
A 30-minute call: you tell us about your application, then we demo the solution for it.