dB/Octave & dB/Decade

An Octave is where the frequency halves or doubles (by a factor of 2) i.e. 400Hz to 800 Hz 

A Decade is where the frequency increases or decreases by a factor of 10 i.e. 5 Hz to 50 Hz 

Slopes can be defined as dB/Octave or dB/Decade

20dB/decade ≈ 6.0205999132796 dB/octave 

There are 3.3 octaves in one decade .. in other words 

6dB/octave is equivalent to 20 dB/decade
           

number of decades