What Is Leq? Equivalent Continuous Sound Level

Leq (equivalent continuous sound level) is a single decibel value that represents the constant, steady-state sound level which would deliver the same total sound energy, over a given measurement period, as an actual fluctuating noise does over that same period. It is not an average of decibel numbers in the ordinary arithmetic sense; it is an energy average, calculated by converting sound pressure back to linear intensity, averaging that, and converting the result back to decibels. Leq is the standard metric used by sound level meters, noise dosimeters, and environmental-noise regulations worldwide to describe noise that rises and falls over time — traffic, construction, industrial machinery, or an open-plan office — as one meaningful number.

Formally, the international sound-level-meter standard IEC 61672-1 (Electroacoustics — Sound Level Meters) defines this quantity with the symbol L_AT (“time-averaged sound level”), noting that it is “commonly referred to as Leq” for historical reasons. That history matters: the concept originated at France’s Laboratoire National d’Essais and was adopted internationally because a single Leq number could finally answer the practical question that a raw, jumping decibel readout could not — “how much total noise energy did this person or place actually receive?”

Why Do We Need an “Equivalent” Sound Level at All?

Real-world noise is almost never constant. A passing truck might peak at 85 dB(A) for two seconds and then fall back to a 55 dB(A) background; a factory floor might cycle between a quiet 70 dB(A) idle state and a 95 dB(A) burst every few minutes. A conventional sound level meter reading such a signal produces a constantly fluctuating number that is impossible to summarize, compare against a legal limit, or use to assess long-term hearing risk.

Leq solves this by asking a different question: not “what is the sound level right now?” but “if this entire time-varying signal were flattened into one constant level for the same duration, what would that level be?” Because sound energy — not sound pressure itself — is what accumulates and what actually damages hearing or disturbs a receiver over time, Leq is calculated from the mean-square sound pressure (proportional to acoustic energy/intensity), not from a simple arithmetic mean of decibel readings. This is the same energy-based logic that governs how you add two decibel levels together: decibels cannot be averaged directly because the scale is logarithmic, so the underlying physical quantity must be averaged first.

How Is Leq Calculated? The Formula Behind the Equivalent Continuous Level

The formal definition of Leq (or LAT) over a stated time interval T is:

L_eq,T = 10 · log₁₀ [ (1/T) ∫₀ᵀ (p(t)² / p₀²) dt ] dB

Where:
– p(t) = the instantaneous sound pressure at time t, in pascals
– p₀ = the reference sound pressure, 20 micropascals (20 µPa), the same reference used for ordinary sound pressure level
– T = the total measurement duration, in seconds

In plain terms: square the instantaneous pressure (which is proportional to instantaneous acoustic intensity), integrate — i.e., sum — that squared pressure continuously over the whole time period, divide by the duration to get an average, and then convert that energy-average back into decibels using the ordinary logarithmic formula. Because there is no exponential time-weighting or decay involved (unlike the F/S/I response of a conventional meter reading), an integrating-averaging sound level meter must be used to measure true Leq — it mathematically sums every instant of sound exposure rather than displaying a smoothed, momentary reading.

The Simplified (Discrete) Version — Combining Known Segments

In practice, noise exposure is often broken into discrete segments of known level and duration (for example, separate work tasks in a shift, or separate traffic conditions in an hour). For n segments, each at level Lᵢ for duration tᵢ out of a total period T, the equivalent formula is:

L_eq = 10 · log₁₀ [ Σ (tᵢ / T) · 10^(Lᵢ/10) ]

This discrete form follows directly from the same logarithmic-addition principle explained in What Is a Decibel? — each segment’s decibel value is converted back to a linear intensity ratio, weighted by the fraction of total time it occupies, summed, and reconverted to decibels.

Worked Example: Calculating an 8-Hour Leq

Suppose a worker spends 2 hours near a machine producing a steady 90 dB(A) and the remaining 6 hours of an 8-hour shift in a 50 dB(A) background environment. The 8-hour Leq is:

L_eq,8h = 10 · log₁₀(250,075,000) ≈ 84.0 dB(A)

This result is instructive: even though the worker spent 75% of the shift in a quiet 50 dB(A) environment, the 2 hours at 90 dB(A) dominate the total energy so completely that the 8-hour Leq comes out at roughly 84 dB(A) — just a few decibels below the loud segment itself, and far closer to 90 dB(A) than to 50 dB(A). This is the same behavior demonstrated by the “10 dB ≈ 10× intensity” relationship covered in the decibel scale guide: a short burst of high-energy noise can dominate an entire averaging period.

What Is the Difference Between Leq and LAeq?

Leq on its own is a generic term for any equivalent continuous level, regardless of frequency weighting. LAeq specifically denotes the equivalent continuous level using A-weighting — the frequency-weighting curve that approximates human hearing sensitivity, described in dB(A) vs dB(C) vs dB(Z): Frequency Weighting. Because nearly all hearing-conservation regulations (NIOSH, OSHA) and most environmental-noise ordinances measure and report levels in A-weighted decibels, “Leq” in occupational and environmental contexts almost always means LAeq in practice, even when written without the “A.” A time period is usually appended as a subscript — for example, LAeq,8h for an 8-hour occupational exposure or LAeq,1h for a one-hour environmental noise reading — since an Leq value is meaningless without stating the averaging period it was calculated over.

What Is “Short Leq,” and How Do Modern Sound Level Meters Measure It?

Rather than storing one single Leq value for an entire shift or day, most modern integrating-averaging sound level meters and noise-monitoring stations record a continuous series of very short Leq values — commonly at intervals as brief as 1/8 second — a technique known as short Leq. Each short-interval value is stored digitally, allowing the complete time history of a noise event to be reconstructed, recombined into any longer averaging period after the fact, or reprocessed under a different regulatory rule without needing to re-measure. Airport noise-monitoring networks, for instance, almost universally rely on short Leq streamed continuously to a central processing system. This method traces back to the original French concept of Leq and is now standardized within IEC 61672, the international standard governing sound level meter design (see How to Measure Sound: Sound Level Meters for meter classes and calibration practice).

Leq vs Lmax, Lmin, L10, L90 — What Do the Other Noise Indices Mean?

Leq is one member of a family of statistical noise descriptors, each answering a different question about the same fluctuating signal. Confusing these indices is one of the most common errors in noise reporting.

Leq and these percentile/statistical levels (L10, L90) are complementary, not interchangeable: two locations can share an identical Leq while one has a steady moderate hum (small gap between L10 and L90) and the other has occasional very loud events against a quiet background (large gap between L10 and L90) — a distinction that matters enormously for annoyance and sleep-disturbance assessment, covered further in How Noise Affects Sleep, Focus & Health.

What Is Sound Exposure Level (SEL), and How Does It Differ From Leq?

Sound Exposure Level (SEL, also written LAE) is closely related to Leq but serves a different purpose: instead of describing a level averaged over an arbitrary period (like 8 hours or a full day), SEL compresses the total acoustic energy of a single, discrete noise event — a single aircraft flyover, one train passing, a single gunshot — into the level it would produce if all of that same energy were compressed into a standard reference duration of exactly 1 second. SEL is calculated as:

SEL = L_eq,T + 10 · log₁₀(T)

where T is the actual duration of the event in seconds. Because SEL references a fixed 1-second window regardless of how long the actual event lasted, it allows fair energy-based comparison between events of very different durations — for example, a brief, loud siren versus a longer, quieter passing train — something a simple Leq comparison over mismatched time periods cannot do.

What Is Lden and Ldn (Day-Night Level)? How Do They Relate to Leq?

Lden (“day-evening-night level”) and Ldn/DNL (“day-night level”) are both 24-hour composite metrics built directly from Leq values for different times of day, with an added penalty applied to evening and nighttime noise to account for greater sensitivity to disturbance and sleep interference during quieter hours.

Lden, the metric used across the European Union (formalized under the EU Environmental Noise Directive, 2002/49/EC) and referenced in ISO 1996-1:2016 (Acoustics — Description, measurement and assessment of environmental noise), is calculated as:

L_den = 10 · log₁₀ [ (1/24) · (12 · 10^(L_day/10) + 4 · 10^((L_evening+5)/10) + 8 · 10^((L_night+10)/10)) ]

Ldn (or DNL), the U.S. equivalent used by the Federal Aviation Administration for airport noise analysis under Federal Aviation Regulation Part 150, uses only a day and night split — a +10 dB penalty applied to nighttime hours (22:00–07:00), with no separate evening category. The FAA has established that an Ldn above 65 dB is generally considered incompatible with residential land use. A related California-specific variant, CNEL (Community Noise Equivalent Level), adds an evening period like Lden but applies a 4.77 dB penalty rather than 5 dB. Exact clock-hour boundaries for these periods may vary slightly by jurisdiction.

The key conceptual link back to this article: Lden and Ldn are not separate physical quantities — they are simply weighted combinations of ordinary Leq values calculated for each period of the day, logarithmically recombined using the same energy-summation math shown in the worked example above. For more on how noise levels are described for public-health purposes, see What Is Noise Pollution? Causes & Health Effects.

How Is Leq Used in Occupational Noise Exposure? NIOSH vs OSHA

In workplace hearing conservation, Leq (calculated as an 8-hour time-weighted average, sometimes abbreviated TWA) is the core metric behind exposure limits — but the two major U.S. frameworks apply different exchange rates, the number of decibels by which the permitted exposure time is halved.

The U.S. National Institute for Occupational Safety and Health (NIOSH) recommends an exposure limit (REL) of 85 dB(A) averaged over 8 hours, using a 3 dB exchange rate (the internationally favored “equal-energy” rule, matching the same physics used to calculate Leq itself). The U.S. Occupational Safety and Health Administration (OSHA), under 29 CFR 1910.95, sets a permissible exposure limit (PEL) of 90 dB(A) over 8 hours with a less conservative 5 dB exchange rate. Personal noise dosimeters used to measure real-world worker exposure — specified internationally by IEC 61252 and in the U.S. by ANSI S1.25 — can typically be configured for either exchange rate.

These figures illustrate why the exchange rate matters so much: under NIOSH’s stricter 3 dB rule, permitted exposure time halves with every 3 dB increase (consistent with 3 dB representing a doubling of sound energy — see What Is a Decibel?); under OSHA’s 5 dB rule, the same halving only occurs every 5 dB, permitting substantially longer exposure at a given level. Full detail on safe daily exposure budgets is covered in How Loud Is Too Loud? Safe Decibel Levels and What Is Noise-Induced Hearing Loss (NIHL)?.

Myth vs Fact: Does a Lower Leq Always Mean a Safer or Quieter Environment?

Myth: “If Location A has a lower Leq than Location B, Location A is always the quieter, less disturbing, or safer place to be.”

Fact: This is not necessarily true. Because Leq is an energy average across an entire period, two very different noise environments can produce an identical Leq value. A steady 65 dB(A) hum sustained for an hour and an environment that is silent for 55 of those 60 minutes but produces a single 5-minute burst averaging over 75 dB(A) can both average out to a similar Leq — yet the second environment, with its sharp intermittent peaks, is typically far more disruptive to sleep, concentration, and conversation, and may involve individual peak levels well above what the Leq number alone suggests. This is precisely why acousticians supplement Leq with Lmax, L10/L90, and SEL (defined above) rather than relying on Leq in isolation — a single averaged number, by design, discards information about how noise was distributed in time.

Why Does Leq Matter in Environmental and Building Acoustics?

Beyond occupational hearing conservation, Leq (and its day/night derivatives, Lden and Ldn) is the standard metric behind virtually all environmental noise regulation and mapping: highway and airport noise impact studies, noise ordinances for construction and nightlife, and the EU’s strategic noise mapping requirements under ISO 1996-1/1996-2 all report exposure as an Leq or Lden figure precisely because it condenses hours (or a full 24-hour day) of constantly changing outdoor noise into one legally usable, comparable number. Building acousticians also use Leq when assessing whether mechanical systems (HVAC, elevators) or road-traffic intrusion into a room meet design criteria — a single sustained Leq figure, rather than a fluctuating raw meter reading, is what gets compared against a target design level.

Related Reading

Leq is one piece of the broader units-and-measurement toolkit covered across this glossary. Start with the physical basis of the decibel and sound pressure level, see how sound intensity and sound power vs. sound pressure relate to what a meter actually measures, and explore the full physics context in Sound & Acoustics Fundamentals: The Complete Guide. For measurement instruments and calibration practice, see How to Measure Sound: Sound Level Meters; for hearing-health implications, see Safe Decibel Levels, Noise-Induced Hearing Loss, and What Is Noise Pollution?

If you are trying to interpret a noise report, an environmental impact statement, or a dosimeter printout and see an unfamiliar Leq, LAeq, Lden, or SEL figure, this glossary is meant as a free, ad-free reference point — not a substitute for a qualified acoustic consultant’s assessment of your specific site or exposure situation.

Frequently Asked Questions

Is Leq the same as an average decibel reading?
No. Leq is an energy average, not an arithmetic average of decibel numbers. Because decibels are logarithmic, simply averaging a series of dB readings arithmetically gives an incorrect (too low) result — the readings must first be converted back to linear intensity, averaged, and then converted back to decibels, exactly as shown in the worked example above.

What does LAeq,T actually mean when I see it written that way?
“LA” indicates A-weighting, “eq” indicates it is an equivalent continuous level, and the subscript T states the averaging period — for example, LAeq,8h means the A-weighted equivalent continuous level averaged over 8 hours, and LAeq,1h means the same calculation over a 1-hour period. An Leq value without a stated time period is incomplete, since the same fluctuating noise produces a different Leq depending on how long you average it over.

Can Leq be higher than every individual level in the measurement?
No. Leq can never exceed the highest instantaneous level recorded during the period, and it can never fall below the lowest. It always falls somewhere between Lmin and Lmax, weighted heavily toward whichever levels contributed the most acoustic energy — as the worked 8-hour example above shows, that is usually the loudest, not the most frequent, segment.

Why is a 3 dB exchange rate considered more conservative than a 5 dB exchange rate?
Because a smaller exchange rate cuts permitted exposure time in half more quickly as level increases. Under NIOSH’s 3 dB rule, exposure time must halve every 3 dB; under OSHA’s 5 dB rule, it only halves every 5 dB — meaning OSHA permits meaningfully longer exposure durations at high sound levels than NIOSH considers safe for hearing conservation.

Is Leq measured differently from ordinary sound pressure level (SPL)?
Both use the same underlying reference pressure (20 µPa) and the same base decibel formula, but ordinary SPL from a standard sound level meter reflects a smoothed, momentary (time-weighted) reading, while Leq requires a true integrating-averaging meter that mathematically sums squared pressure continuously over the entire stated period with no time-constant smoothing.

Does a 24-hour Lden of 55 dB mean the noise was always at 55 dB throughout the day?
No. Like Leq, Lden is a single composite figure built from an energy-weighted combination of separate day, evening, and night Leq values (with evening and night levels penalized upward before combining), so a stated Lden reflects the overall 24-hour exposure profile, not a constant, unchanging sound level.