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 (the reference pressure)
has a sound pressure level of 0 dB, while 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, and has developed recommended
guidelines for noise/land use compatibility evaluation. |
