SOUND DESIGN WORKSHOP: Notcher Daddy's Flanger

Learning about moving notch filters can take flanging to a new level.
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In the November 2006 “Sound Design Workshop: Sounds in Motion” (available online at, Jim Aikin showed several creative ways to use multiple filters to add motion to your sounds. One of my favorite uses for multiple filters is to create a cascade of moving notch filters. That produces an effect reminiscent of phasing and flanging, but it is both subtler and more flexible.

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FIG. 1: Four instances of Volcano are set up as modulated notch filters, then arranged in series for a simulated flanging effect.

I'll use multiple instances of FabFilter Volcano ( for my examples, but any effects plug-in or virtual instrument that can process audio and has lowpass and highpass filters that you can arrange in parallel will do the trick. Volcano consists of a pair of filters in series or parallel, and you can modulate the filters' parameters independently.

The Setup

Each notch in the cascade requires one instance of Volcano. In each Volcano, arrange the filters in parallel with one in lowpass mode and the other in highpass mode. In that case, any frequency passed by either filter appears at the output, so the cutoff frequency (labeled FREQ) of the lowpass filter should be the same or slightly lower than that of the highpass filter. The result is a band of frequencies, called a notch, that is reduced in level by the combined filters. You don't want the notch to be very wide because that will carve out too much of the signal when you place several of the filters in series to form the cascade. Even using equal cutoff frequencies produces a notch because the signal is attenuated 3 dB at a filter's cutoff frequency.

The slope of the filters (12, 24, or 48 dB per octave in Volcano), which indicates how sharply the level falls after the cutoff frequency, controls the depth of the notch. Any of these slopes works well when the cutoff frequencies are equal, and the slopes don't need to match. The resonance setting (labeled Peak) will also have an effect, with high settings producing an audible ringing near the cutoff frequency. As with the slope, the Peak settings need not match.

In Volcano, you can pan the filters independently, but it's not a good idea to do that because it defeats the notch effect. However, it is useful to pan both filters to the same position when using several Volcanos in series. Modulating the pan with one of Volcano's LFOs adds motion to the effect.

It's About Time

Volcano has two LFOs and an ADSR envelope generator for modulation. The LFOs have ramp and pulse waveforms with variable width and can be free-running or synced to tempo. You set whether MIDI Note messages or audio above a certain threshold triggers the envelope generator. Bipolar modulation knobs set the amount of each modulator applied to cutoff, resonance, and pan for each filter. As with flangers and phasers, motion is key, but a little modulation goes a long way.

If your source material is percussive, the threshold-triggered envelope makes a great modulator. For pads and ambient tracks, use a MIDI drum part to trigger the envelope. Kick drum parts work well for imposing a little rhythm on a pad. Setting the envelope to modulate the two filter cutoffs a small amount in opposite directions makes the notch width pulsate with the kick drum rhythm.

I like to use a moderate-rate triangle-wave LFO to modulate both cutoffs by a small amount with the same polarity. I simultaneously use a long automation envelope to sweep both cutoffs slowly across a wider frequency range. The same trick works well with pan modulation.

As an alternative to automation envelopes, I often assign the same MIDI controller to both Freq or both Pan knobs. Volcano's MIDI Learn implementation doesn't allow that, but many plug-in hosts do.

With modulation, a single Volcano imparts significant motion, but arranging several in series, with their notches at different frequencies and with different modulation setups, magnifies the effect (see Fig. 1 and Web Clip 1). That uses more CPU, so you may need to freeze the track after setting up the effect.

Len Sasso is an associate editor of EM. For an earful, visit his Web site