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Audio comb filtering is the pattern of repeating frequency boosts and cancellations that occurs when a sound combines with a delayed copy of itself. The delay changes the copies’ relative phase at different frequencies, creating a response with peaks and notches like the teeth of a comb.
How a delay creates comb filtering
When a sound and its delayed copy are combined, their waveforms may line up at some frequencies and oppose each other at others. Signals that line up reinforce one another; signals that arrive half a cycle apart partially or fully cancel. Because the phase relationship changes as frequency changes, the result alternates between peaks and dips.
The delay sets the spacing and positions of those peaks and notches. In the University of California, Irvine’s Max Cookbook example, a 1 ms delay reinforces whole-number multiples of 1,000 Hz and suppresses frequencies halfway between them, including 1,500 Hz. This is an illustrative example, not a measured statistic.
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Where audio comb filtering occurs
Room reflections
At a listener or microphone, sound may arrive along a direct path and then along a longer reflected path. Those arrivals are separated in time, so their combination can create a comb-shaped frequency response. Dynaudio’s acoustics appendix describes the response produced by direct and reflected sound; Shure’s guidance on acoustic interference also covers the phenomenon in sound systems.
Multiple microphones
When two microphones capture the same source from different distances, the sound reaches each at a different time. Summing their recordings can create frequency peaks and dips. The depth of the effect depends in part on the relative levels of the signals, as well as their timing relationship. Shure discusses microphone placement and isolation, including gobos, in its recording guidance.
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Intentional delay effects
Comb filtering is not always a problem to eliminate. Flanging deliberately combines a signal with a delayed copy. When the delay changes, the peaks and dips move through the spectrum, producing the characteristic sweeping sound. Sound On Sound’s explanation of comb filtering discusses this effect and its recording applications.
How to investigate an unwanted effect
First identify the delayed path rather than treating every case as a mixing problem. The likely source may be a room reflection, another microphone capturing the same sound, or a delay effect in the signal chain.
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- If it comes from a reflection: change the source, microphone, or listening position, or reduce the relevant reflection. Treatment depends on the room and recording goal.
- If multiple microphones are involved: adjust their placement, isolate sources where practical, or align the recorded tracks if that suits the recording.
- If processing created it: change or remove the delay if the moving or hollow coloration is not wanted.
These options address different causes; there is no single placement or processing adjustment that fits every situation.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Why a polarity switch is not a universal fix
A polarity switch reverses a waveform’s sign. A time delay instead creates a phase difference that varies with frequency. Inverting polarity can cancel a matching signal in some circumstances, but it does not remove the frequency-dependent timing relationship created by a delay. Sound On Sound notes that a console or preamp “phase” button usually inverts polarity; it is not a continuously adjustable phase control.
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