“Lover of the low-high Q, let's hear your view”
If you recognize the above quote from an album, then it's probably time to update your musical preferences!
However, the reference is clearly to the option that many equalizers have of setting a low or a high Q, or anything in between.
Firstly, what is Q?
The concept of Q in equalizers primarily applies to mid-band EQ sections. Raising the level of a particular band of mid frequencies creates a bell shape in the frequency response curve.
This bell shape can be wide and cover a wide range of frequencies (low Q), or narrow and cover only a small range of frequencies (high Q).
So Q therefore represents the sharpness of the resonance that creates this bell-shaped curve.
If you want a technical description, then Q = f0/(f2 – f1) where f0 is the center frequency of resonance, f2 and f1 are the upper and lower frequencies that are 3 dB lower in level than the center frequency.
Q is a ratio and therefore is just a number; it has no units. A Q of 1 or less would be considered a low Q, 5 or higher is a high Q. But which do you use?
Generally the best way to use Q is to set a low Q if you want to do frequency shaping to improve the overall subjective sound of a signal. Set a Q of 1 or less, then set the frequency and gain controls of the EQ according to what sounds good to you.
If you have a problem however, such as an unwanted resonance in an acoustic guitar, then set a high Q, set a cut in gain, then adjust the frequency control to find the resonance.
Generally it's good to set either a low or high Q first, then set the other controls, then fine tune the Q as necessary.