9/29/09 D-5
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
Note: The latter is frequency and level dependent, but the
value “10 dB” is a good rule of thumb, especially around 1
kHz.
Table 1 shows the actual value of a specific item, such as
sound power, for which the sound level is calculated. First,
the sound power value is divided with the reference used and
then the ten-based logarithm is applied. This value is then
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