The decibel (dB) is a unit used to measure sound intensity
and other physical quantities. A decibel is one tenth of a bel (B), a unit named
after Graham Bell, the inventor of the telephone. Its logarithmic scale is
convenient to represent the entire range of human hearing.
The decibel of sound pressure level (dB SPL) takes as a
reference the minimum sound pressure level that the average human ear can
detect. The smallest audible sound to humans is typically 0 dB SPL (hearing
threshold). In practice, “dB” often stands for “dB SPL”.
Because the decibel scale is logarithmic, a three-decibel increase in sound
level already represents a doubling of [sound] intensity. For example, a normal
conversation may be about 65 dB and someone shouting can typically be around
80dB. The difference is only 15 dB but the shouting is 30 times as intensive.
Please note that perception of loudness is not exactly the
same as sound pressure level. To account for the fact that are particularly low
and high-pitched sounds appear less loud to the human ear, noise is usually
measured in A-weighted decibels (dB(A)).
It is not just the intensity that determines whether noise is hazardous. The
duration of exposure is also very important. To take this into account,
time-weighted average sound levels are used. For workplace noise, this is
usually based on an 8-hour working day.
The equivalent continuous sound pressure level over eight
hours (Lequ,8h) is the sound intensity level,
expressed in dB(A), which in the course of continuous exposure for 8h would
result in the same amount of sound energy received as that from the actual