Sound exposure is the integral, over time, of squared sound pressure. The SI unit of sound exposure is the pascal squared second (Pa²·s).

Sound exposure, denoted E, is defined by

${\displaystyle E=\int _{t_{0}}^{t_{1}}p(t)^{2}\,\mathrm {d} t,}$

where

• the exposure is being calculated for the time interval between times t₀ and t₁;
• p(t) is the sound pressure at time t, usually A-weighted for sound in air.

Sound exposure level (SEL) is a logarithmic measure of the sound exposure of a sound relative to a reference value.
Sound exposure level, denoted L_E and measured in dB, is defined by^[1]

${\displaystyle L_{E}={\frac {1}{2}}\ln \!\left({\frac {E}{E_{0}}}\right)\!~\mathrm {Np} =\log _{10}\!\left({\frac {E}{E_{0}}}\right)\!~\mathrm {B} =10\log _{10}\!\left({\frac {E}{E_{0}}}\right)\!~\mathrm {dB} ,}$

where

• E is the sound exposure;
• E₀ is the reference sound exposure;
• 1 Np = 1 is the neper;
• 1 B = 1/2 ln 10 is the bel;
• 1 dB = 1/20 ln 10 is the decibel.

The commonly used reference sound exposure in air is^[2]

${\displaystyle E_{0}=400~\mathrm {\mu Pa^{2}\cdot s} .}$

The proper notations for sound exposure level using this reference are L_{W/(400 μPa²⋅s)} or L_W (re 400 μPa²⋅s), but the notations dB SEL, dB(SEL), dBSEL, or dB_SEL are very common, even if they are not accepted by the SI.^[3]