Short answer: Leq (equivalent continuous sound level) is the steady level that would carry the same sound energy as a fluctuating noise over the same period. It is an energy average, not an arithmetic one, so loud periods dominate: one hour at 80 dB plus one hour at 60 dB gives an Leq of about 77 dB over the two hours, not 70 dB.
Noise reports, workplace assessments and planning conditions are full of terms like LAeq,T, L90 and Lden. They all describe the same varying sound in different ways. This guide explains what Leq is and how to read the notation, works through an example, sets out the statistical levels L10, L90, Lmax and Lpeak, covers the day-evening-night ratings used for environmental noise, and shows how to measure them yourself.
What Leq Means
Real noise rarely stays still: traffic rises and falls, machines cycle on and off, people talk and stop. Leq condenses that into one number, the constant level that would deliver the same total energy over the measurement time:
Leq = 10 · log10( (1/T) · Σ ti · 10Li/10 )
where each level Li lasts for time ti and T is the total time. Because the decibel scale is logarithmic, each level is converted to energy before averaging. The result is always closer to the loudest periods than a simple average would be.
Reading the Notation: LAeq,T
- L stands for level, in decibels.
- A is the frequency weighting: A-weighting, which follows the ear at moderate levels (see dB vs dBA). LCeq uses C-weighting, which shows bass better (see dBA vs dBC).
- eq means equivalent continuous level.
- T is the period: LAeq,15min, LAeq,1h, LAeq,8h or LAeq,16h (daytime), for example.
So “LAeq,15min = 100 dB”, the WHO limit for music venues, means the A-weighted energy average over 15 minutes is 100 dB.
Worked Example: Why Loud Periods Dominate
One hour at 80 dB followed by one hour at 60 dB: the arithmetic average is 70 dB, but the Leq over two hours is 10·log10((108 + 106) ÷ 2) ≈ 77 dB. The quiet hour contributes only 1% of the energy.
A working day example: 2 hours at 88 dB(A) and 6 hours at 75 dB(A) give LAeq,8h ≈ 82.6 dB(A). That is below the NIOSH limit of 85 dB(A), but the two loud hours alone use half the daily allowance. You can check these sums with the average tab of the decibel calculator or enter each task in the noise exposure calculator.
L10, L90 and Other Statistical Levels
Leq tells you the energy; statistical levels describe how the noise is distributed over time. LN is the level exceeded for N% of the measurement time.
| Metric | Meaning | Typical use |
|---|---|---|
| L10 | Level exceeded 10% of the time | The noisier moments; used for road traffic noise (for example LA10,18h in the UK) |
| L50 | Level exceeded half the time | The median level |
| L90 | Level exceeded 90% of the time | Background noise between events; used for background in UK BS 4142 assessments |
| Lmax | Highest time-weighted level | Single events: a passing lorry, a door slam, a dog barking |
| Lmin | Lowest time-weighted level | The quietest moment |
| Lpeak | Highest instantaneous pressure, not time-averaged | Impulse noise: gunshots, hammering, fireworks |
| Lden, Ldn | Leq over 24 h with evening and night penalties | Environmental noise maps and guidelines |
Lmax and Lmin depend on the time weighting: Fast (125 ms) or Slow (1 s). Lpeak is usually C-weighted and far higher than Lmax for sharp sounds; OSHA limits impulse noise to 140 dB peak, and EU Directive 2003/10/EC uses peak action values of 135 and 137 dB(C) with a limit of 140 dB(C), levels well above 120 dB.
A quiet street at night might read L90 35 dB, Leq 45 dB and Lmax 70 dB when a car passes. The gap between L10 and L90 shows how intermittent a noise is, which is often what makes it annoying, and is why neighbour noise complaints often cite Lmax as well as Leq.
Lden and Ldn: Day-Evening-Night Levels
Noise at night disturbs more than the same noise at midday, so environmental ratings add penalties. Lden, used under the EU Environmental Noise Directive (2002/49/EC), is a 24-hour Leq with +5 dB added to the evening (4 hours) and +10 dB to the night (8 hours). Ldn, common in the US, adds +10 dB to night-time (10 pm to 7 am) and has no evening band. The WHO Environmental Noise Guidelines (2018) recommend road traffic noise below 53 dB Lden and 45 dB Lnight, close to 55 dB and 45 dB on our scale.
Why Regulations Use Leq
Hearing damage depends broadly on total sound energy received, the “equal energy principle”, so Leq is the natural measure of exposure. In Europe, daily exposure is expressed as LEX,8h, the LAeq normalised to an 8-hour day, with action values of 80 and 85 dB(A) and an exposure limit of 87 dB(A). The NIOSH recommendation of 85 dB(A) for 8 hours with a 3 dB exchange rate is also an equal-energy rule; OSHA’s 5 dB exchange rate is not. See the OSHA and NIOSH exposure guide, why 85 dB is the danger line, what 80 dB means over a day, and the safe noise levels chart.
Room criteria use LAeq too: recommended levels for offices and classrooms are normally stated as LAeq over a working or teaching period.
How to Measure Leq, L10 and L90 with Our Tools
The noise monitor records a session and reports Leq, L10, L90 and Lmax, which is enough for most home and workplace checks. For a quick background reading (effectively L90), use the background noise test; for a live reading, use our sound decibel meter. Run long measurements at the times the noise matters, and note events that affect Lmax. Phone microphones are good for about 40–100 dB once calibrated; reports for planning or legal use need a Class 1 or Class 2 instrument (see sound level meter vs decibel meter). Our guide on how to measure decibels covers positioning and timing.
Frequently Asked Questions
What does LAeq mean?
The A-weighted equivalent continuous sound level: the energy average of a varying noise over a stated time, weighted to match hearing.
Is Leq the same as the average dB?
It is the energy average, not the arithmetic one. Loud periods count for much more, so Leq is always equal to or higher than the arithmetic mean of the readings.
What is the difference between Leq and Lmax?
Leq averages the energy over the whole period; Lmax is the single highest time-weighted level. A quiet room with one door slam has a low Leq and a high Lmax.
What is a good L90 for a bedroom?
Quiet bedrooms at night typically have a background around 25–35 dB(A). Compare your readings with our typical room noise levels.
What does LAeq,15min mean?
The A-weighted Leq over 15 minutes. The WHO safe listening venue standard sets 100 dB LAeq,15min as the maximum.
Addito is an audio analysis and sound measurement writer specializing in decibel testing, noise level analysis, sound frequency measurement, and audio engineering tools for creators, audio professionals, students, and everyday users. He creates practical content focused on sound measurement, hearing safety, microphone testing, and acoustic analysis tools.