root-mean-square amplitude (rms amplitude) has different formulas based upon the shape of the signal. For sine waves only it is given by:
Decibels is a comparison between two signals.
When comparing amplitude use this formula:
When comparing the power use this formula:
Sometimes decibels are presented as units. This is because their is an implicit signal that is being used for comparison. Here are some common ones:
a) dBV: 1 volt rms
b) dBm – the voltage corresponding to 1mW with an assumed load impedance
c) small noise generated by a resistor at room temperature