Why an EQ cannot fix a cancellation

What is actually happening

Sound reaches your microphone by more than one route. The direct path from the speaker, plus at least one more via a wall, the floor, the stage edge, or a second speaker. The second route is longer, so it arrives later.

At the frequencies whose half wavelength fits exactly into that path difference, the two arrive in opposite phase and cancel. Between them they add. The result is not one notch but a row of them at regular spacing — a comb filter. A 1.70 m path difference puts the first null at roughly 100 Hz, the next at 300, then 500.

Why boosting achieves nothing

An EQ sits in front of the speaker. All it can do is make the one signal louder, and that signal then travels both routes — the direct one and the delayed one. Both get louder by the same amount. Their ratio to each other does not change, so they keep cancelling exactly as before.

What does change is everything else: at that frequency the amplifier delivers twelve times the power, the cone moves accordingly, the limiter engages sooner, and everywhere in the room where the notch is not, it is now too loud. At the measurement point you gain one or two decibels — the cancellation eats the rest.

How to recognise it

• It moves. Step half a metre sideways and measure again — if the notch has shifted or vanished, it was a cancellation. A genuine resonance stays where it is.
• It comes in a series. A single notch can be anything. Several at even spacing are a comb filter, and a comb filter always has a delay behind it.
• It is narrow and deep. Room resonances and speaker faults are usually broad and rarely deeper than six or eight decibels. A 15 dB drop across a third of an octave is almost never something a filter can reach.
• It does not go away. You add 6 dB, measure again, and the notch is still there. That is the most honest test there is — and the one too few people run.

What helps instead

• Change the path difference. Move the speaker half a metre, take the sub out of the corner, get the microphone off the floor. That shifts the notches to where they hurt less.
• Delay instead of equalise when two sources are involved. If the second route is not a wall but the sub, it is not a room problem but a timing problem — and timing problems are solved with delay and polarity, not with an EQ.
• Make the second route quieter. A blanket over the stage edge, an absorber at the first reflection point, a different coverage angle. The weaker the second route, the shallower the notch.
• Leave it alone. Not every notch has to go. One that exists at a single seat in the room is heard by almost nobody — and fighting it makes things worse for everyone else.