Larson Davis 831C-FFT, Справочное руководство

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SoundAdvisor Model 831C C-4
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 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.
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
Table C.1 Sound Level
Power form, squared units Level form
Ration of Value to
Reference
Exponential Form
of Ratio
10•Exponent
1
10
0
0
10
10
1
10
100
10
2
20
200
10
2.3
23
1,000
10
3
30
10,000
10
4
40
100,000
10
5
50
1000,000
10
6
60