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When the decibel is used to express SPL, the reference sound pressure is 20 x 10
-6
Newtons/m²
which is approximately the threshold for hearing for a normal listener. When using a dB meter to
measure sound, the meter is performing the calculation between the received SPL and the
reference SPL:
⎟
⎠
⎞
⎜
⎝
⎛
×
=
−6
spl
1020
SPL
log20dB
Equation 2-5. dB and Sound Pressure Levels
When multiple sound sources are combined, there is an increase in SPL. However, you cannot
add decibels directly:
90 dB + 90 dB is not 180 dB but 93 dB. Doubling the power results in a +3 dB SPL increase.
To add SPL decibels:
1. Convert the decibels back to the original power value (which for SPL, is referenced to 1pW
or 10
-12
W).
2. Add the numbers together.
3. Convert the numbers back to decibels.
To add 90 dB + 90 dB:
()( )( )
93
10
002.0
log10dB
W002.0W001.0x2P2
W001.0
101010
P
10
P
10
;
10
P
log9
10
P
log1090
W
10
pW1
P
;
P
P
log10dB
12
3129
12
9
12
12
0
0
12
=
⎟
⎠
⎞
⎜
⎝
⎛
=
==
===
⎟
⎠
⎞
⎜
⎝
⎛
=
⎟
⎠
⎞
⎜
⎝
⎛
=
⎟
⎠
⎞
⎜
⎝
⎛
=
==
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
=
−
−−
−
−
−
−
Equation 2-6. Adding Decibels
Basic Audio Math, Continued
Ohm’s Law
and the Decibel,
(continued)
Adding Decibels