A 1/3-octave graphic EQ has Q≈4.31, not the 100 that engineers often assume

Engineers frequently assume that a graphic equalizer provides narrow, surgical filter control — as if each band has a high Q (narrow bandwidth). The measured reality is that a standard 1/3-octave GEQ has a filter bandwidth of 1/3 octave, which corresponds to Q≈4.31. Adjacent filters overlap substantially. This means: (1) pulling one fader also affects frequencies one and two bands away; (2) the GEQ cannot target the specific feedback frequency an engineer hears (e.g., 839.2Hz) — the closest band may be at 800Hz or 1kHz, and pulling it will notch a wide region, not a narrow spike. This is not a design flaw in cheap GEQs; it applies to all 1/3-octave designs.

Examples

A GEQ with 31 bands from 20Hz to 20kHz has each band separated by roughly 1/3 octave. If feedback appears at 1.4kHz, the nearest bands are 1.25kHz and 1.6kHz — both distant from the exact frequency, and pulling either will affect a wide swath of the 1–2kHz region.

Assessment

Given a 1/3-octave GEQ and a feedback frequency of 2.8kHz, identify the closest band(s), estimate the resulting Q, and explain why notching this band may not eliminate the feedback.