Larson Davis SoundAdvisor 831C Reference Manual page 206

SoundAdvisor Model 831C
Note: The latter is frequency and level dependent, but the value "10 dB" is a good
rule of thumb, especially around 1 kHz.
Table C.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
multiplied by 10 to create the decibel value (see equation below).
For every 10 decibels, a unit called Bel is created. The decibel stands for: deci for
"one tenth" and bel for "Bel" (compare decimeter). The relationship between Bel
and decibel is thus: 1 Bel = 10 decibels. It is not possible to directly add or
subtract decibel values, since addition of logarithmic values correspond to
multiplication of the original quantity.
Table C.1 Sound Level
Power form, squared units
Ration of Value to
Reference
1
10
100
200
1,000
10,000
100,000
1000,000
Each time the sound pressure level increases by 6 dB, the corresponding sound
pressure value is doubled and thus multiplied by 2. Each time the sound power
level increases by 3 dB, the sound power value is multiplied by 2. Thus, it is
important to notice that a doubling of the sound power is equal to 3 dB, and a
doubling of the sound pressure is equal to 6 dB, since a doubling of the sound
pressure will result in a quadruple increase of the sound power. The advantage
with using dB is simply that they remain the same even if we use sound pressure
or sound power. Compare this to the use of voltage and power units in electrical
engineering, units being related by P~V2. In table 2 an illustration is made of
values calculated on sound pressure, non-squared units.
The original definition of decibel was intended for power-like quantities, such as
sound power. If we consider sound pressure levels instead (usually denoted P in
acoustics), the equation will be the same, since the "two" in the squared units will
move from within the bracket and become a 20 log instead of a 10 log and thus
compensate for using linear or quadratic units. Please note that it is not allowed
to use 20 log for squared units, since that expression assumes that we use linear
units, like sound pressure in acoustics or voltage in electrical engineering. This is
illustrated in equation below:
Exponential Form of
Ratio
0
10
1
10
2
10
2.3
10
3
10
4
10
5
10
6
10
Level form
10•Exponent
0
10
20
23
30
40
50
60
C-4