You add a second mic to a source, or move a vocalist closer to a music stand, and the sound suddenly thins out: it goes hollow, “telephone-like”, as if scooped out through the middle. The mic is not bad and neither is the room: this is comb filtering. It is one of the most common phenomena in recording and mixing, and one of the easiest to fix once you understand the mechanism.
What is comb filtering?
As soon as two copies of the same signal combine with a small time offset, they interfere. At some frequencies the two waves are in phase and add up: the level rises. At others they are opposed and subtract: the level collapses. These boosts and dips repeat at regular intervals across the spectrum. Plot the resulting frequency response and it looks like the teeth of a comb — hence the name.
The key point is that the spacing of the dips depends only on the time offset. A very short delay pushes the first dip high up the spectrum; a longer delay pulls the dips down and multiplies them right where it matters, in the range of voices and instruments. A few tenths of a millisecond are enough for the ear to hear the colour change.
A concrete marker: with a one-millisecond delay (1 ms), the first dip lands around 500 Hz, and the next ones repeat roughly every 1,000 Hz. Double that to 2 ms and the dips drop and tighten, right in the vocal range. Since sound travels about 34 cm per millisecond, a few centimetres of difference between two mics is enough to colour the tone noticeably. That is the whole gap between an offset you chose and one you suffered.
Where it hides in the studio
Comb filtering shows up anywhere a sound arrives twice, slightly offset:
- Two mics on one source. The textbook case: a pair on an acoustic guitar, two mics on a snare, or a close mic plus a room mic. The two capsules are not the same distance away, so the sound reaches them offset.
- A mic and a reflection. Even with a single mic, the direct sound combines with what bounces off the floor, a stand, a window or a nearby wall. The reflection is a delayed copy: comb filtering guaranteed.
- A miked amp and a parallel DI. The signal from the mic chain arrives after the DI’s, which is near-instant. Mixed together, they comb.
- Headphone bleed into an open mic, or the same signal picked up by several mics on a drum kit.
How to hear and spot it
The telltale symptom is a thin, metallic or “in-a-jar” tone that changes the moment you move the mic or mute one of the two sources. The simplest test: listen to each source alone, then together. If the combination sounds hollower than one source on its own, you have your culprit. To see it, a spectrum analyser (RTA) clearly shows the series of regular dips; a plain mono check, listening to the sum, also reveals the cancellations that stereo hides.
How to avoid it at the source
Prevention always beats correction. A few reflexes:
- The 3-to-1 rule. When you use two mics on a source, place the second at least three times farther from the first than the first is from the source. The offset becomes large enough to push the comb out of the sensitive range.
- Move reflective surfaces away or treat them. An absorption panel on the floor or behind the singer kills the reflection before it returns. That is one job of acoustic treatment.
- Mind your placement. Our notes on positioning a vocal mic and on miking techniques serve the tone and the comb problem at once.
How to fix it in the mix
When the damage is done, three levers:
- Check the polarity. Flipping the phase of one of the two mics can turn a catastrophic dip into a boost. It is not magic, but it is the first move to try.
- Time-align the tracks. In the DAW, nudging a track by a few samples so the transients coincide removes the offset at source. Tools such as automatic phase alignment do this on drums.
- EQ sparingly. A touch of equalization can tame the worst peaks, but EQ does not “fill in” a cancellation dip: treat the cause, do not paper over the symptom.
The same phenomenon exists in sound reinforcement: two speakers covering the same area comb in the room. That is why live system alignment is largely about managing those overlaps.
My take
Comb filtering has a bad name it does not deserve: handled well, the offset between two mics is also a colour tool, and you can hear it on countless records. The reflex worth building is the mono check: it lays the cancellations bare before they wreck a mix. And when you are starting out, one well-placed mic will almost always beat two placed at random. Here, simplicity is a strategy, not a compromise.
