In every kind of dB, a factor of 10 in amplitude increase corresponds to a 20 dB boost (increase by 20 dB):
and , of course. A function which is proportional to is said to ``fall off'' (or ``roll off'') at the rate of dB per decade. That is, for every factor of in (every ``decade''), the amplitude drops dB.
Similarly, a factor of 2 in amplitude gain corresponds to a 6 dB boost:
and
A function which is proportional to is said to fall off dB per octave. That is, for every factor of in (every ``octave''), the amplitude drops close to dB. Thus, 6 dB per octave is the same thing as 20 dB per decade.
A doubling of power corresponds to a 3 dB boost:
and
Finally, note that the choice of reference merely determines a vertical offset in the dB scale: