Comb filtering is the pattern of regular peaks and dips spread across the frequency range that shows up when a sound is mixed with a slightly delayed copy of itself. Some frequencies get louder and others get cut, in a repeating, evenly spaced pattern that runs up the spectrum. Plot it and it looks like the teeth of a comb, which is where the name comes from.
It is a specific result of phase interference between a signal and a delayed copy of it, so it is related to phase but more particular. The causes and fixes here are standard and work in any DAW, and none of it is tied to one product.
Comb filtering is the pattern of regular peaks and dips spread across the frequency range that appears when a sound is mixed with a slightly delayed copy of itself. Add the two together and some frequencies are reinforced while others are cancelled, in a repeating, evenly spaced pattern that runs all the way up the spectrum. Drawn on a graph, that alternating row of peaks and dips looks like the teeth of a comb, which is where the name comes from.
It happens because of how the two copies line up at each frequency. A fixed delay is a different fraction of the cycle for every frequency, so at some frequencies the delayed copy arrives in step with the original and the two add up into a peak, while at others it arrives out of step and the two partly subtract into a dip. Move up the frequency range and the copies fall back into step, then out again, then back in, so peaks and dips alternate at even spacing. A shorter delay spreads the teeth further apart; a longer delay packs them closer together.
Comb filtering is a specific result of phase interference between a signal and a delayed copy of it. It is closely related to the wider idea of phase, but narrower: phase is where a waveform sits in its cycle, while comb filtering is the particular comb-shaped set of peaks and dips you get when a sound and a delayed version of it are summed. Where a single phase problem might thin out one region, a comb rakes the whole spectrum with a regular row of notches.
Anything that adds a slightly delayed copy of a sound back to itself. The delay involved is usually tiny, far too short to hear as an echo, but long enough to set up the comb. A handful of everyday situations cause most of it.
| Cause | Where the delayed copy comes from | What helps |
|---|---|---|
| Two mics on one source | The sound reaches the far mic a moment after the near one, so summing the two tracks stacks a delayed copy against the original. | Use the three-to-one spacing rule, or time-align the two tracks after recording. |
| A mic near a wall or floor | The direct sound and its reflection off the nearby surface reach the mic a short time apart and combine at the capsule. | Move the mic away from the surface, or damp the surface so the reflection is weaker. |
| A doubled or short-delayed track | A part copied and shifted by a very short amount, or a short delay mixed back in, sits a fraction behind the dry signal. | Line the copy up with the original, or lengthen the delay until it reads as an echo instead. |
| A stereo pair folded to mono | Left and right carry the same sound with a small timing difference, which only stacks up when the two are summed to mono. | Check in mono, tighten the timing between the channels, or keep the low end centred. |
The thread through all of them is the same: one sound arriving twice, a short moment apart, and the two copies summed together. That is worth holding onto, because it is also the whole basis of an effect people reach for on purpose.
Comb filtering is not always a fault. A flanger is deliberate comb filtering that moves: it mixes a sound with a very short, constantly sweeping delayed copy of itself, which builds a comb of notches and then slides them up and down the spectrum to give that whooshing, jet-plane sweep. The difference is control and motion. A flanger is a comb you dial in and sweep for effect, while the comb filtering in this guide is the accidental, static kind that colours a sound without your asking.
Comb filtering usually sounds hollow, metallic or phasey, and in milder cases boxy, coloured or a little robotic. Because it scoops out a regular row of evenly spaced bands and lifts the ones between them, the sound loses its natural balance and takes on a fixed, resonant character that was not in the source.
How strong it is depends mostly on how loud the delayed copy is against the original. When the copy is nearly as loud as the dry sound, the notches are deep and the colouring is obvious, often thin, glassy, or like singing through a tube or a pipe. When the copy is much quieter, a distant reflection rather than a second close mic, the effect is subtle, heard more as a faint hollowness or a sense that the tone is slightly off than as an obvious effect. Because the pattern sits still, your ear tends to read it as a property of the sound rather than as something added, which is part of why it can be easy to miss until you go looking for it.
Keep a sound from arriving twice a short moment apart, and check for it in mono so you can hear it when it does. The common moves are the three-to-one mic spacing rule, time-aligning or nudging tracks that drifted apart, moving a mic away from a reflective surface, and folding the mix to mono to reveal a comb that hides in the stereo spread.
The three-to-one rule is the usual starting point when you record one source with more than one mic. Place the second mic at least three times as far from the first mic as the first mic is from the source. The idea is that the source is much quieter by the time it reaches the second mic, so when the two tracks are summed the delayed copy is low enough to keep the combing mild. It is a rule of thumb rather than a guarantee, but it heads off the worst of it at the source.
After recording, the fix is usually to time-align or nudge. If two tracks of one sound are offset because it reached them at different moments, slide one until the two line up, which removes the delay that made the comb. Flipping the polarity of one track can help as well, though on its own it shifts the notches around rather than removing them, so treat it as a partial fix rather than a cure.
For a reflection, move the mic, or move the source, away from the nearby wall, floor or desk, or put something absorbent on the surface so the reflected copy is weaker. And whatever the cause, check in mono. Comb filtering that hides across a stereo image jumps out the moment the two sides are summed, because that is exactly the operation that stacks the copies together, so a quick mono check is the fastest way to catch it before it reaches a listener.
Most comb filtering is decided before you touch a fader. A single, focused source that reaches the mix once, with no second copy trailing a fraction behind it, has nothing to comb-filter against. The less you stack near-identical copies of a sound, the less of this you ever have to hear or repair.
We do not sell a tool that removes comb filtering, and this page is tool-agnostic on purpose. But comb filtering almost always creeps in at the recording stage, from two mics on one source, a nearby reflection, or a part doubled against itself. The Collection is our three instruments together, ARGISH, SILT and REHEAT, with three separate licence keys. Each one gives you a single, clean source that you place once where you want it, with no second mic and no stray delayed copy stacked against it, so there is far less chance of a comb forming in the first place. ARGISH is a self-playing chord synth, SILT is a tape-loop instrument, and REHEAT writes acid lines. Every one ships as AU, VST3, AAX and a standalone app on macOS, signed and notarized, and a VST3 on Windows.
You can hear one free in your browser first, no install and no account, then decide.
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