The Decibel (db)
A measure of the sound pressure of a given noise source relative to a standard reference value is the sound pressure level (SPL). This reference pressure is typical of the quietest sound that a young person with good hearing is able to detect and is measured in decibels (dB). The logarithmic conversion of sound pressure to sound pressure level means that the quietest sound we can hear has a sound pressure level of 0 dB. The loudest sounds we can hear without pain have sound pressure levels on the order of 120 dB. Most sounds in our day-to-day environment have sound pressure levels on the order of 30 to 100 dB.
The Day-Night Average Sound Level (DNL)
In simple terms, Ldn or DNL is the average noise level over a 24 hour period. Except that noise occurring at night (between the hours of 10PM and 7AM) are artificially increased by 10 dB. This weighting reflects the added intrusiveness of night noise events attributable to the fact that community background noise typically decreases by 10 dB at night.
Under Federal Aviation Regulation (FAR) Part 150. The FAA has established Ldn/DNL as the cumulative noise exposure metric for use in airport noise analyses. They have also developed recommended guidelines for noise/land use compatibility evaluation.