^ I think I could be a little clearer concerning decibels.
A decibel is a way to express a value relative to another reference value, it is mathematically defined a 10 log ( Value / (Reference Value)), thus when the value you measure is equal to the reference value, it becomes 0 dB.
Decibels are used in a lot of situations.
- to describe sound pressure levels, in this case the reference value for 0 dB is the limit of audibility, the unit is dB SPL.
- voltage ratios, if the reference voltage is 1 V, it's dBv, if it's 0.775 V, it's dBu
- to describe a level relative the maximum level possible, it's often used for digital signals where the max level is 0 dBFS (for Full Scale) and all other level are relative to this 0 dBFS, noise for 16 bit audio is at -96 dBFS.
Anyway, the important thing to remember when thinking about decibels is that you are always considering a ratio.
(NB: For field units, it's 20log(V/Vref) instead of 10 log, there's a reason for that, but don't worry about it foo much for now)