Phase and polarity: why a mix sounds hollow and how to fix it

The symptom before the theory

There's one fault that doesn't resemble any other: the mix sounds fine and is somehow hollow. You push the kick and the low end doesn't grow. You bring in the second snare mic and the snare gets smaller, not bigger. You double a guitar to thicken it and the result has less body than the single track. In every one of those cases you're summing two signals that are eating each other.

That's a phase problem, and it has one quality that makes it especially frustrating: it doesn't sound like a defect. There's no click, no distortion, no offending frequency you can point at. Just an absence. And because it doesn't sound bad, the usual reaction is to reach for EQ and boost precisely what's cancelling — which is exactly what doesn't work.

Two words that don't mean the same thing

Most of the confusion comes from using «phase» for both things. They're worth separating, because they're fixed in different ways.

What happens when they sum

The three situations that matter, with the two signals on top and their sum underneath:

In practice you're never exactly in the first case or the second. The normal one is the third, repeated across the whole spectrum: a fixed delay shifts low frequencies a little and high frequencies a lot, so some reinforce and others cancel at the same time. That's what comb filtering is, and it's why a badly aligned double sounds metallic and hollow instead of thick.

Where it actually shows up

• Multiple mics on one source. Kick in and out, snare top and bottom, amp close and room. It's the classic case and the most audible, because the two signals are nearly identical.
• DI and amp on a bass or guitar. The DI arrives first; the amp has to travel through air to the mic. One or two milliseconds is enough to thin the low end.
• Overheads and spot mics. The kit reaches the overheads later than the close mics, and the snare usually goes odd when they sum.
• Doubles and layered samples. Two stacked kicks starting at slightly different moments subtract instead of adding.
• Parallel processing. A copy of the signal down another chain comes back with that chain's latency. If the DAW doesn't compensate, the sum cancels.
• Very wide stereo effects. Wideners create differences between channels; summing to mono cancels part of that by definition.

How to spot it in thirty seconds

Diagnosis is easier than the fix, and it only takes a button you already have.

• Sum to mono. If the mix thins noticeably, something is cancelling. A healthy mix loses width in mono, not body.
• Isolate in pairs. Bring up two related tracks — the two kick mics, the DI and the amp — and push one up and down. If raising it makes the pair smaller, that's your culprit.
• Try the Ø. With both playing, hit invert on one. Keep whichever position has more low end and more body, without theorising about which one it «should» be.
• Look at the waveforms. Zoom into the attack of a hit: if one peak goes up while the other goes down at the same instant, it's polarity. If both go up but offset, it's time.

How to fix it, in order

Cheapest first. It's almost always solved in the first two steps.

• Flip the polarity. One click, reversible, no side effects. Always start here.
• Align in time. Drag the track until the attacks line up. Between mics on the same source this is the right move; zoom in, don't eyeball it at bar scale.
• Move the microphone. Only available while you're still recording, but it's the one solution that fixes the cause instead of the symptom.
• All-pass filter. Shifts the phase of some frequencies without touching level. It's the fine tool for when flipping improves one region and worsens another — typical between kick and bass.
• Drop one of the two. If two mics won't get along and none of the steps above helps, one well-chosen mic usually beats two fighting.

What isn't a phase problem

It's worth knowing when to stop looking. If two elements occupy the same region of the spectrum and cover each other, that's masking, and it's solved by separating them in frequency or time, not by flipping anything — see the kick and bass guide. And if a mix sounds small overall but doesn't change when summed to mono, the problem is in levels or gain staging, not in phase.