Larson Davis SoundAdvisor 831C Reference Manual page 201

SoundAdvisor Model 831C
• 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 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:
dB = 10Log
Table C.1 illustrates how a a tenfold increase of the sound pressure will result in an increase in 20
dB steps, while sound power increases in 10 dB steps. See the linear form (Table C.1) and compare
with equation above. In conclusion, dB values are always the same, independent of using sound
power or sound pressure as the base unit. A 6 dB increase implies four times the sound power or
two times the sound pressure.
Exponential Form
of Ratio
0
10
1
10
2
10
2.3
10
3
10
4
10
5
10
6
10
2
P P
------- - ----- -
= 20Log
10
2
P
P
0
0
Level form
10•Exponent
0
10
20
23
30
40
50
60
p ; = 20Pa
0
C-4