The Q of an EQ

Q sets how wide a boost or cut reaches, and the same curve can sound like two different moves.

Related

EqualizerPianoVoice

The question

Lift one frequency and change only Q, and the result moves from a thicker range to one pitch calling out. Q sets how much spectrum the boost occupies, not how much boost there is.

How it works

Q is the centre frequency divided by the bandwidth, measured between the minus 3 dB points. A higher Q narrows the band and concentrates the phase shift near the centre. On a bell filter, Q and gain shape the curve together, and a narrow high gain rings more. On a shelf, Q changes how abrupt the transition is, not which frequency is chosen.

Settings and what you hear

A wide Q near 0.5 to 1 thickens a range. Values from 2 to 4 address a specific resonance. Above 8 suits hunting a whistle. A narrow boost adds a pitch-like character; a wide one feels like another layer. Sweep to find the problem, then widen to decide how much to remove.

What to listen for

Sweep a violin or voice from 200 Hz to 800 Hz, keep the boost fixed and change only Q. A narrow setting hums suddenly as it crosses one frequency; a wide one thickens the passage. On low piano notes, narrow ringing sounds like an extra drone.

Common misconceptions

That a higher Q is always better; that Q changes width but not phase; that a narrow cut has no side effects.

Sources

• Bristow-Johnson, Cookbook Formulae for Audio EQ Biquad Filter Coefficients — formulas checked, not quoted
• Udo Zölzer, DAFX — Digital Audio Effects, filters chapter — background checked, not quoted

Used for fact-checking only; all prose is original.