Online Comb Filter: Learn How Delay Creates Peaks and Notches Across the Spectrum

Comb filtering is one of those effects that's easy to name and hard to actually hear happening — a diagram of interference in a textbook doesn't tell you what a phasing sound is until you've heard one form. A comb filter is created by summing an audio signal with a copy of itself delayed by a few milliseconds: the result is a series of cancellations and boosts across the frequency spectrum, producing the characteristic "metallic" or phasing sound.

With this tool you can adjust the delay in milliseconds and hear the effect applied to different sample tracks, while watching the waveform in time and the frequency spectrum side by side — the fastest way to connect what you hear with what's actually happening to the signal. Part of AudioFxLab's free collection of interactive audio tools.

Parameter Guide: How Each Control Works

Delay (ms) / Distance (cm)

A comb filter works by mixing a signal with a delayed copy of itself. The delay time determines which frequencies get reinforced and which get cancelled: at any frequency whose wavelength relates to the delay time by a whole number of cycles, the two copies arrive in phase and reinforce each other; at frequencies that fall exactly between those points, the copies arrive out of phase and cancel out. The result, on the frequency-domain display, is a series of evenly spaced peaks and notches that resembles the teeth of a comb — which is where the effect gets its name.

Because sound travels through air at a known, fixed speed (343 m/s), any delay time also corresponds to a physical distance — this tool shows both figures linked together (1 ms ≈ 34.3 cm) so you can think of the effect either as a delay time or as the distance a reflected sound would travel to arrive that many milliseconds late, which is exactly what happens acoustically when a reflected sound off a wall, a nearby surface, or a second microphone reaches a listener or microphone slightly after the direct sound.

Where to start: very short delays, well under 1 ms, a few centimeters, produce widely spaced notches that mostly affect the highest frequencies; longer delays, several milliseconds, tens of centimeters, pack the notches closer together across the whole audible spectrum, producing the more pronounced "phasing" or "metallic" character the effect is generally associated with.

Phase (Normal / Inverted)

This flips the polarity of the delayed copy before it's summed with the original — inverting it 180°. Since a comb filter's notches and peaks come from constructive and destructive interference between the two copies, inverting the polarity of one of them swaps which frequencies cancel and which reinforce: the notches of the normal setting become the peaks of the inverted setting, and vice versa.

Where to start: compare the frequency-domain display in both settings at the same delay time — the pattern of peaks and notches shifts by exactly half a cycle, a direct, visible demonstration of what phase inversion does to interference.

Bypass

This switches the display and playback between the comb-filtered signal, original plus delayed copy, and the unprocessed original signal alone, letting you compare directly what the effect is doing, both by ear and by looking at how the waveform and spectrum differ from the untouched source.

Master Volume

Overall playback level, independent of the filtering parameters.