The term Q is short for "Quality" and really relates to the reactance of a capacitor or inductor, the higher the Q the better the "quality" of these reactive components. Somehow it found it's way into audio terms.

But Q is really just a way to express bandwidth, and the Q can be found by the formula Q=f/BW where f = the centre frequency and BW = the -3dB points either side of the centre frequency.

So, if you have a filter centred at 1000Hz, and the -3dB points are 2000Hz and 500Hz (a difference of 1500Hz), the Q will = 0.6666. (1000/1500).

Filter types:

1. High-pass and low-pass filters
2. Shelving filter
3. Graphic equalizer
4. Parametric equalizer