What room EQ can and cannot fix

A dip on a frequency response can mean two entirely different things. Either the system is producing less energy at that frequency, or it is producing plenty and two copies of it are arriving at your ears out of phase and subtracting.

EQ can fix the first. It cannot fix the second, and the reason is worth internalising because it applies everywhere: when you turn up a frequency, you turn up the direct sound and the reflection equally. Their ratio does not change, so the cancellation does not change. What changes is how hard your amplifier and woofer are working to produce a result that looks identical.

• Modal peaks. A room mode is a resonance storing energy and releasing it slowly. Cutting the input to a resonance lowers both its level and how long it rings, so a cut here fixes the waterfall as well as the frequency response. This is the strongest case for EQ that exists.
• Broad tonal offsets. Too much upper bass, not enough presence, a subwoofer set 4 dB hot. Wide, gentle, consistent at every seat, and exactly what a shelf filter is for.
• Speaker voicing you disagree with. A designer's choice about the balance is a broad shape, and broad shapes EQ cleanly.

• Speaker boundary interference. Sound reflecting off the wall behind a speaker returns delayed by twice the wall distance. Where that delay is half a wavelength, it cancels. The frequency is set by the distance and nothing else, which also means moving the speaker moves the problem, and is free.
• Seat nulls. Every room mode has planes where the air does not move. Sit on one and that frequency is genuinely absent at your ears, however hard the speaker works. Moving the seat is the fix, and the tool on this site will calculate where to move it.
• Reverberation time. Decay is how slowly energy leaves the room. A filter changes how much energy goes in. These are different quantities and no filter setting connects them.
• Comb filtering above the modal region. It is different at every point in space, including at your two ears. Correcting for one point makes the others worse.

This is where most setups go wrong even when the individual steps are right. EQ is fitted to the room as it is at the moment you measure. Change anything physical afterwards and the correction is now fitted to a room that no longer exists.

1. 1.Move the seat and the speakers. Free, reversible, and typically the largest single improvement available in an untreated room.
2. 2.Trap the bass. Corners first, because that is where every mode has a pressure maximum and where an absorber therefore does the most work.
3. 3.Treat the first reflection points. This is where imaging and clarity come from, and it is not something EQ addresses at all.
4. 4.Measure again.
5. 5.Now apply EQ, to whatever is left.

Done in this order, the EQ has less to do, uses smaller filters, and holds up across a wider area of the room. Done in the reverse order, you will redo it three times.