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What is sound exposure level (SEL)?
Sound exposure level, SEL or LAE, is the level of a steady one-second sound carrying the same acoustic energy as a whole event. A 10-second flyover averaging 80 dBA has an SEL of 90 dB, because ten seconds of energy squeezed into one second is 10 dB higher. It lets a short loud event and a long quieter one be compared, and it is the building block of airport noise metrics.
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The formula
SEL = LAeq,T + 10 log₁₀(T / 1 s), where LAeq,T is the A-weighted average level over the event and T is its duration in seconds. Doubling the duration of an event adds 3 dB to its SEL. Because it normalises to one second, SEL is always at least as high as the maximum level of any event longer than a second, which surprises people reading airport reports.
| Average level | 1 s | 10 s | 30 s | 60 s |
|---|---|---|---|---|
| 70 dBA | 70 | 80 | 84.8 | 87.8 |
| 80 dBA | 80 | 90 | 94.8 | 97.8 |
| 90 dBA | 90 | 100 | 104.8 | 107.8 |
Sources: FAA, Fundamentals of Noise and Sound; ISO 1996-1:2016, Acoustics, description and assessment of environmental noise.
Where SEL is used
- Aircraft noise. Each flyover is logged as an SEL; summing the events over a day and averaging gives the day-night average level, DNL, with a 10 dB penalty on night events. The FAA uses DNL 65 dB as its significance threshold.
- Rail and industrial events. ISO 1996 uses SEL for single events such as train passes and blasts.
- Sleep research. Awakening studies relate the chance of waking to the SEL of an event rather than its peak.
For a continuous source, the OSHA and NIOSH exposure limits use a different quantity, the time-weighted average, which spreads energy over an 8-hour day instead of one second.
What is the difference between SEL and Lmax?
Lmax is the loudest moment of an event. SEL is the total energy of the event expressed as one second. Two flyovers with the same Lmax can differ by 5 dB in SEL if one lasts longer.
What is the difference between SEL and Leq?
Leq is the average level over a chosen period; SEL is that average adjusted to a one-second reference. For a one-second event they are identical. For anything longer, SEL exceeds Leq by 10 log of the duration in seconds.
How is DNL calculated from SEL?
Convert each event SEL to energy, add 10 dB to events between 10 pm and 7 am, sum the energies over 24 hours, then average over the 86,400 seconds of the day. The result is the day-night average level the FAA and HUD use for land-use decisions.
Does a decibel meter show SEL?
Integrating sound level meters do, usually labelled LAE. A phone meter shows the instantaneous level and sometimes Leq; you can approximate SEL from Leq and the event length with the formula above.