The roll-off rate is the slope offilters, expressed in decibels(dB) per octave or per decade. For example, if a filter has a -3 dB drop at 200 kHz and an attenuation of 27 dB at 400 kHz, then the roll-off rate is 24 dB/octave.
The roll-off rate changes near the roll-off frequency, it can be increased by the polarity of filters. For example, if a single-pole filter has a roll-off of 20 dB/decade, then the roll-off is 40 dB/decade for a double-pole, 60 dB/decade for a 3-pole, and so on. Note that a frequency response roll-off of 20 dB/decade corresponds to one of 6 dB/octave.
In addition to the roll-off rate, there is also the roll-off frequency. This is the frequency at an amplitude drop of 3 dB. The terms roll-off or roll-off rate are mainly used for frequency response curves, but they can also be used for other level-dependent changes of a physical quantity, for example in loudspeakers.
For microphones, we speak of low-end roll-off and high-end roll-off. Microphones with a low-end roll-off have a gradual decrease in frequency response at low frequencies, i.e. bass frequencies. The situation is different for microphones with high-end roll-off. With these microphones, the sensitivity gradually decreases at high frequencies above 15 kHz. A slowly decaying roll-off is characterized by a natural sound.