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Audio Comb Filter

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Often followed by + Reverb + Equalizer + Limiter

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Settings are written to the URL as you change them. Nothing differs from the defaults yet.

Ring a tuned delay against the signal

A comb filter delays the signal by a few milliseconds and adds it back. At that length the delay is too short to be heard as an echo and is heard as a pitch instead: frequencies whose period divides evenly into the delay reinforce, and the ones between them cancel.

Tuning is set in Hz rather than in milliseconds, because the pitch is what matters. 220 Hz is a delay of about 4.5 ms, and the peaks land on 220, 440, 660 and so on.

Where it sits among the modulation effects

The flanger, the phaser, and this all build on delay, and they do different jobs.

A flanger is a comb filter whose delay is swept by an LFO. Its notches move constantly, which is the whooshing sound, and because they never settle it does not sound like a note.

A phaser uses allpass stages instead of a delay. Its notches are not evenly spaced, so they do not form a harmonic series and the result colours the sound without tuning it.

This one holds still. The delay is fixed, tuned to a frequency, and the feedback is left to resonate, which is what makes it a resonator rather than a modulation effect.

Resonance

Resonance is the feedback around the loop. It decides how long the ring lasts and how strong the peaks are.

  • 0 to 30%: colouration. The peaks are shallow and the sound is tinted rather than tuned.
  • 40 to 70%: an audible pitch that follows the input’s dynamics.
  • 80 to 100%: a sustained ring that keeps sounding after the input stops. A drum hit or a noise burst excites the whole loop and it keeps only the frequencies that fit, which is how a snare turns into a note.

The feedback is bounded below 1 so the loop cannot run away at any setting, including while the tuning is gliding between two frequencies.

Polarity

Peaks resonates at the fundamental and every harmonic. Hollow inverts the sign in the feedback path, which cancels the even harmonics and leaves the odd ones. The perceived pitch drops roughly an octave and the tone moves toward a square wave, which is the same relationship as a stopped organ pipe against an open one.

Damping

Damping is a one pole filter inside the loop rather than on the output, so it compounds pass by pass. Each reflection loses a little more of its top end than the one before it, which turns a buzz into a note that darkens as it fades.

Because it sits inside the loop, damping also shortens the ring. Energy removed on one pass never comes back round on the next.

Two ways to use it

As a resonator, set resonance high and mix around 50%, then tune it to the key of the material. Percussion and noise take on a pitch, and sustained sources gain a strong harmonic.

As a tone shaper, keep resonance under 30% and tune it low. The peaks and notches then act as a fixed pattern of colouration, which is a way to make a source sound like it is coming through a pipe or a small enclosure.

In a chain

Sitting after distortion, a comb filter picks a harmonic series out of the harmonics the distortion generated, which is more interesting than resonating a clean source. Before reverb rather than after, so the ring is placed in the space instead of the space being tuned.

Watch the level at high resonance. A loop this close to unity concentrates energy in narrow bands and the output can peak well above the input, so a limiter after it is worth having.

Frequently Asked Questions

It adds a delay short enough to be heard as a pitch rather than as an echo, then feeds it back on itself. The result rings at that pitch and at whole multiples of it, so the frequency response looks like the teeth of a comb and any input, including noise, comes out sounding tuned.

The flanger sweeps its delay, so its comb is always moving and never settles on a pitch. The phaser uses allpass stages, so its notches are not evenly spaced and do not form a harmonic series. This one is fixed, tuned in Hz, and left to resonate.

Peaks puts resonances on the fundamental and every harmonic above it. Hollow inverts the feedback, which cancels the even harmonics and leaves only the odd ones, roughly an octave lower in perceived pitch and closer to a square or clarinet tone.

Feedback near the top of the range means energy takes a long time to leave the loop, so the delay keeps ringing after the input has stopped. Any broadband sound, a snare or a noise burst, excites every frequency at once and the loop keeps only the ones that fit its length, which is the tuned note.

It puts a filter inside the feedback loop, so the ring loses more of its top end on each pass. Low damping gives a bright, buzzy, almost metallic ring; high damping gives a rounder note that fades faster, because energy the filter removes never comes back round.

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