The confusion

The concept of dB (decibel) is a common source of confusion among audio consumers, mainly because dB's are happily used to describe two different things: voltage levels and power levels.

1. If we are talking about electric voltage or digital sample values (i.e. what you could measure in your EWS64 line in/out connectors or what you would see in your Soundforge level display), the rule of thumb is indeed that a 6dB increase means doubling the original value:  +6 dB  = 2 x  = 200%.
   Example: sample value 2000 is 6 dB higher than sample value 1000.

2. If we are talking about electric power or acoustic power (as with loudspeaker output), the rule of thumb is that a 3dB increase means doubling the value: +3dB = 2 x = 200%.
   Example: a 200 Watt amplifier will give you 3 dB more acoustic power than a 100 Watt amplifier.

In case 1) the formula for dB difference between two values V1 and V2 is 20 * log (V1/V2).
In case 2) the formula for dB difference between two values P1 and P2 is 10 * log (P1/P2).

The 6 dB rule of thumb in case 1) comes from the fact that 20 * log(2 * V / V) = 20 * log 2 = 20 * 0.301 = 6 (dB).
The 3 dB rule of thumb in case 2)  comes from the fact that 10 * log(2* V / V) = 10 * log 2 = 10 * 0.301 = 3 (dB).

As long as we are not dealing with power amplifiers and loudspeakers, we should stick to case 1.
Thus, for us:
+6dB = double the original value = 200%
- 6dB = half the original value = 50 % etc.

This is how SoundForge, CoolEdit and any digital editing software with dB scales work.

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Latest update on page: 08-02-99