+4 dBu / -10 dBV: These values are typically used to represent ‘Pro’ and ‘Consumer’ audio signal levels - some gear even has a switch to scale the output, or to adjust input gain. So what do these values mean on a common scale?
First we need to know how to get from dB to volts. For linear measures like volts, we defined the measure in dB to be 20log10(volts/reference). To go backwards we’ll need volts = reference10^(dB/20).
You’ll recall from a previous post that the reference for dBV is 1 volt, and the reference for dBu is 0.775 volts.
Let’s convert 4dBu to dBV, via volts:
0.775*10^(4/20) = 1.228V
20*log10(1.228/1) = 1.8dBV
So +4dBu=+1.8dBV, and the difference between +4dBu and -10dBV is really about 12 dB.