It simply a measurement derived through the use of a mathematical formula for a specific purpose. The following metrics are those most commonly used in airport noise. These metrics are A-weighted unless otherwise noted.
Ldn (day-night average sound level)
The average sound level over an entire day, with 10 dB being added between 10 pm and 7 am to account for the added intrusiveness of noise during these hours. Ldn contours used for mitigation purposes generally represent an average day that is calculated or modeled using a full year of data.
Leq (equivalent-continuous sound level)
An equivalent-continuous sound level is simply an average level (without 10dB weighting) over a stated time period.
SEL (sound exposure level)
SEL is similar to Leq. The difference between the two is that an SEL represents the entire noise event as though it occurred within a duration of one second. With the SEL metric we can directly compare two noise events having different durations.
Lmax (maximum sound level)
This is the highest level displayed on a sound level meter during a noise event or time period.
Lmin (minimum sound level)
This is the lowest level displayed on a sound level meter during a noise event or time period.