What is Lden and Lnight? A guide to EU noise metrics

Why averages, not peaks

Environmental noise is assessed using long-term average metrics rather than one-off peaks, because it is sustained exposure — not a single loud lorry — that drives annoyance and health effects. Lden and Lnight are the two standard EU indicators, used by Defra and throughout UK planning.

Lden: the day-evening-night level

Lden is an annual average level that weights noise by time of day. Evening noise has a 5 dB penalty added and night-time noise a 10 dB penalty, reflecting that the same sound is more disturbing later in the day. The result is a single figure that captures the overall daily noise climate, biased appropriately towards the more sensitive periods.

Lnight: the sleep metric

Lnight is the simple average level over the night period (23:00–07:00), with no penalties. Because it targets the hours when people sleep, it is the metric most closely linked to sleep disturbance and the associated health effects, and it often decides whether a property near a road is comfortable.

Reading them together

A property can have a moderate Lden but a problematic Lnight if night-time traffic is significant, or vice versa. That is why AcoustaPlot reports both, and sets your risk rating from whichever is worse — so the rating never understates the real impact.

A worked example

Suppose a road produces the same steady level all day and night. Lden would come out a little above that raw level, because the evening and night hours carry penalties of 5 and 10 decibels respectively. Lnight, by contrast, simply reports the night-time average with no penalty. In practice traffic is lighter at night, so Lnight is usually lower than Lden — but where night-time flows stay high (for example on a freight route), the gap narrows and the night metric becomes the one to watch.

How they map to health thresholds

The WHO recommends road-traffic exposure below 53 dB Lden and 45 dB Lnight. UK policy adds the SOAEL concept — the significant observed adverse effect level — above which mitigation is expected. Reporting both metrics lets you see not just whether you are 'loud' overall, but specifically whether night-time exposure crosses the thresholds most associated with sleep and health effects.

Why AcoustaPlot reports both

Because a property can be comfortable on one metric and problematic on the other, AcoustaPlot always reports both Lden and Lnight and sets your risk rating from whichever is worse. That way the headline rating can never hide a night-time problem behind an acceptable daytime average.

The problem these metrics solve

Environmental noise varies constantly — louder at rush hour, quieter at night, punctuated by individual vehicles — so a single instantaneous reading tells you very little about what it is like to live somewhere. Regulators needed a way to summarise a whole year of this variation in a small number of figures that correlate with how noise actually affects people. Lden and Lnight are the answer: standardised, long-term average indicators adopted across the EU and used by Defra and throughout UK planning.

Because they are standardised, they also allow fair comparison — between properties, between roads, and against health-based thresholds — in a way that ad-hoc measurements never could.

How the day-evening-night weighting works

Lden stands for 'day-evening-night level'. It takes the average noise across a year but applies penalties to the more sensitive periods: an extra 5 decibels are notionally added to evening noise (typically 19:00–23:00) and an extra 10 decibels to night noise (23:00–07:00) before averaging. The effect is that noise occurring in the evening or at night counts for more in the final figure than the same noise during the day, reflecting the greater disturbance it causes.

So Lden is not simply the average sound level you would measure — it is a weighted index designed to track annoyance, deliberately biased towards the hours when noise matters most. A property with significant evening and night traffic will have a higher Lden than its raw average would suggest.

Lnight and its link to health

Lnight is simpler: it is the straightforward average level over the eight-hour night period, with no penalties applied. Its purpose is narrow and important — to capture the noise during the hours people sleep, because sleep disturbance is one of the best-evidenced health effects of environmental noise. The WHO's night-noise guidance and much of the epidemiological research on noise and cardiovascular risk are framed around Lnight, which is why it carries so much weight.

In practice Lnight is usually lower than Lden, because traffic thins overnight. But on routes with heavy night-time flows — major freight corridors, roads near 24-hour facilities — the gap narrows, and Lnight becomes the figure that determines whether a home is genuinely peaceful.

Reading the two together

The real value comes from reading Lden and Lnight side by side. A property might have a moderate Lden but a problematic Lnight if night traffic is significant, or a high Lden driven by heavy daytime and evening flows with a comparatively quiet night. Each pattern implies a different lived experience and a different set of priorities — the first points towards protecting sleep, the second towards daytime amenity and glazing.

This is why AcoustaPlot always reports both metrics rather than collapsing them into one number, and why it sets your headline risk rating from whichever of the two is worse. That way the rating can never mask a night-time problem behind an acceptable daytime average, or vice versa.

From metric to decision

For a homeowner or buyer, the practical use of these metrics is to place your property on a health-based scale. Compare your Lden against the WHO road-traffic guideline of 53 dB and your Lnight against 45 dB; sitting comfortably below both suggests noise is unlikely to be a significant issue, while sitting well above points to a closer look at mitigation and, for high night-time figures, at sleep specifically. UK policy adds the SOAEL concept — the level above which effects are significant and mitigation is expected — as a further benchmark.

Understanding what Lden and Lnight mean turns an intimidating pair of acronyms into a straightforward tool: two numbers that, read against clear thresholds, tell you where your property stands and whether it is worth acting.

A brief history of the metrics

Lden and Lnight were introduced across the European Union by the Environmental Noise Directive as common indicators, so that noise could be mapped and compared consistently between countries and sources. Before their adoption, different places used different measures, making comparison awkward. Standardising on these two indicators — one capturing the weighted whole-day picture, the other focused on sleep — gave regulators, planners and researchers a shared language for environmental noise. The UK retains them in its own mapping and policy, which is why they appear on Defra maps and in AcoustaPlot reports.

Knowing that these are deliberately designed, internationally standardised indicators — rather than arbitrary numbers — helps make sense of why they are weighted and averaged the way they are.

Other metrics you might encounter

Lden and Lnight are the strategic indicators, but you may meet others in professional contexts. LAeq expresses an equivalent continuous level over a stated period and underlies much building-design guidance such as BS 8233; LA10 and LA90 describe the levels exceeded for 10% and 90% of the time (roughly, the 'louder' and 'background' levels); and Lmax captures the single loudest event, relevant for sleep disturbance from individual pass-bys. Each answers a slightly different question, which is why a full acoustic assessment may use several.

For a homeowner, the two strategic metrics are usually sufficient to understand exposure and risk; the others become relevant mainly in detailed professional work such as a measured acoustic survey.

Reading your own figures with confidence

Armed with what Lden and Lnight mean, you can read an assessment with confidence. Check the daytime figure for overall amenity and the night figure for sleep; compare each against the WHO guideline (53 and 45 dB respectively) and note how far above or below you sit; and take the worse of the two as your headline, since that is the one that governs your risk. Add the honest uncertainty range, and you have a clear, defensible read on where your property stands.

That is precisely how AcoustaPlot presents its results — both metrics, benchmarked against the thresholds, with the rating set by the worse of the two — turning two technical acronyms into a straightforward, actionable picture of the noise at your home.

The takeaway

Lden and Lnight are simply standardised, long-term average noise indicators — one weighted across the whole day to track annoyance, the other focused on the night to track sleep effects. Read together and compared against the WHO guidelines of 53 and 45 decibels, they place a property clearly on a health-based scale, and taking the worse of the two as the headline ensures a night-time problem is never hidden behind an acceptable daytime average.

That is exactly how AcoustaPlot presents its results, turning two technical acronyms into a straightforward, actionable read on the noise at your home.