D-4 Glossary Model 831 Manual
during sleeping hours. The same occurs from 10 p.m. to
midnight. They are energy summed and converted to an
average noise exposure rating.
Decibel (dB) A logarithmic form of any measured physical quantity and
commonly used in the measurement of sound and vibration.
Whenever the word level is used, this logarithmic form is
implied. The decibel provides us with the possibility of
representing a large span of signal levels in a simple manner
as opposed to using the basic unit Pascal for acoustic
measurements.
It is not possible to directly add or subtract physical
quantities when expressed in decibel form since the addition
of logarithmic values correspond to multiplication of the
original quantity.
The word level is normally attached to a physical quantity
when expressed in decibels; for example, Lp represents the
sound pressure level.
The difference between the sound pressure for silence versus
loud sounds is a factor of 1,000,000:1 or more, and it is very
unpractical to use these large numbers. Therefore, a measure
that would relate to “the number of zeros” would help, for
example, 100,000 would be equal to 50 and 1000 would be
equal to 30 and so on. This is the basic principal of the dB
measure.
All dB values are unit free and therefore, the dB value is not
the value of the quantity itself, but the ratio of that quantity
to an actual reference quantity used. Thus, for every level in
decibels there must be a well defined reference quantity.
Sound versus vibration uses different references, but the dB
principal is the same. When the quantity equals the reference
quantity the level is zero. To keep dB values above zero, the
reference is generally set to be the lowest value of the
quantity that we can imagine or normally wish to use.
Before explaining the calculation of dB values, it is useful to
remember the following rules of thumb when dB values are
used for sound levels:
- Doubling of the Sound Pressure = 6 dB
- Doubling of the Sound Power = 3 dB
- Doubling of the Perceived Sound Level = (approx) 10 dB
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