Comb filtering: causes and cures
When a signal is added to a delayed copy of itself, some frequencies reinforce and others cancel. The frequency response looks like the teeth of a comb, hence the name.
Where the notches fall
For a delay of τ seconds, cancellation occurs at
f = (2k + 1) / (2τ), k = 0, 1, 2, …
A delay of 1 ms puts the first notch at 500 Hz, then at 1.5 kHz, 2.5 kHz and so on. Peaks fall halfway between, at multiples of 1/τ.
How deep the notches are depends on the relative level. Equal levels cancel completely; if the delayed copy is 6 dB quieter, the notches are only about 6 dB deep. Use the comb filter visualizer to see this.
Where you meet it
- Two microphones on one source, for example a snare top and bottom, or several drum microphones picking up each other.
- Microphone near a surface: the reflection off a table or floor is a delayed copy of the direct sound.
- Loudspeakers that are not time-aligned, or two speakers covering the same area.
Avoiding it
- Reduce the level of the delayed copy. The 3:1 rule says: keep the second microphone at least three times as far from the source as the first, so that leakage is about 9.5 dB lower.
- Time-align the signals by delaying the earlier one.
- Check polarity. Inverting one channel shifts the notch pattern and can improve or worsen a given situation.
- Use a boundary microphone to place the capsule so close to the surface that the reflection is effectively in phase.
Draft: written by an AI assistant, awaiting technical review.
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