[Boatanchors] decibels (was Microphone Output)

[David Stinson]

"dB" is the unit of measure of a relative logarithmic ratio,
usually involving power or amplitude.
It needs a suffix to define a point of reference 
to which the reading is relative.

A few examples of these units are:
dBm = power relative to 1 mW dissipated in 600 ohms.
dBW = power relative to 1 W regardless of load impedance.
dBi =  gain of an antenna relative to an isotropic radiator.
dBd = gain of an antenna relative to a dipole in free space.
dBV =  voltage relative to 1 volt, regardless of impedance.

A chart of the ratios and more info than you want is at:
http://en.wikipedia.org/wiki/Decibel