IEC 61672-1: Equivalent Continuous Level (Leq) and Sound Exposure Level (SEL)
TL;DR
Leq is the constant sound pressure level that would deliver the same total energy as the actual fluctuating sound over the measurement period. Defined in IEC 61672-1 Clause 3.8, LAeq is computed as 10·log10[(1/T)∫p²A(t)dt / p₀²] where T is the measurement duration and p₀ is 20 µPa. SEL (Sound Exposure Level) normalizes this energy to a 1-second reference period: LAE = LAeq + 10·log10(T). SEL is useful for comparing events of different durations, such as vehicle pass-bys or aircraft flyovers.
Equivalent Continuous Level — Leq (Clause 3.8)
The time-average sound pressure level Leq represents the steady level that contains the same acoustic energy as the actual time-varying sound measured over period T. For A-weighted measurement:
LAeq,T = 10 · log₁₀ [ (1/T) · ∫₀ᵀ pA²(t) dt / p₀² ]
Where pA(t) is the A-weighted instantaneous sound pressure and p₀ = 20 µPa is the reference pressure. LAeq is the single most important noise metric worldwide — it is referenced by virtually every environmental noise regulation, building acoustics standard, and occupational health guideline.
Practical Interpretation
Leq accounts for both level and duration. A 90 dB event lasting 1 minute contributes the same energy as an 80 dB source lasting 10 minutes. This energy-equivalence principle is the foundation of noise dose calculations per OSHA and ISO standards.
Common measurement periods:
- LAeq,1s — 1-second intervals for logging
- LAeq,5min — typical environmental survey intervals
- LAeq,1h — hourly averages for monitoring stations
- LAeq,8h — 8-hour average for occupational exposure (also called TWA)
- Lden — day-evening-night composite per EU Environmental Noise Directive
Sound Exposure Level — SEL (Clause 3.9)
SEL (symbol LAE) normalizes the total sound energy to a reference duration of 1 second:
LAE = LAeq,T + 10 · log₁₀(T / T₀)
Where T₀ = 1 second. SEL is always greater than or equal to LAeq for measurement periods longer than 1 second. It is the preferred metric for discrete noise events — aircraft flyovers, train pass-bys, explosion blasts — because it allows direct comparison regardless of event duration.
Relationship Between Leq and SEL
If N identical events each with sound exposure level LAE occur in time period T, the total LAeq is:
LAeq,T = LAE + 10 · log₁₀(N / T)
This relationship is used extensively in transportation noise prediction models to calculate Ldn and Lden from per-event SEL values and event counts.
Implementation Considerations
IEC 61672-1 requires integrating-averaging sound level meters to compute Leq with a maximum integration error of ±0.3 dB (Class 1) or ±0.5 dB (Class 2) for a 1 kHz signal. The integration must be continuous — no gaps or dropouts during the measurement period.
SonaVyx Leq and SEL
The SonaVyx SPL meter computes LAeq continuously with 1-second resolution. SEL is calculated automatically when you stop the measurement. The Fast/Slow/Impulse time-weighted levels are computed in parallel with Leq. For peak level measurements, LCpeak is tracked independently. Export your Leq time history as CSV for post-processing in environmental monitoring workflows.
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Last updated: March 19, 2026