Sound Design Workshop: Clean Sweep

CREATE MOVING FILTER EFFECTS WITH SHORT DELAY LINES
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Four delay lines are swept by the LFO in Propellerhead Reason's Subtractor synth to process a sampled string patch in the NN-19 sampler.

The familiar sweeping effect known as flanging takes its name from the way in which it was originally produced using a pair of analog tape machines. The machines were fed the same input, and the signals from their playback heads were mixed and recorded while someone pressed lightly on the flange of the feed reel of one machine to slow it down relative to the other. That caused an increasing delay between the two output signals. Before the delay became recognizable, the pressure was switched to the feed reel of the other machine, decreasing and eventually reversing the relative delay.

In the digital world, it's a simple matter to re-create this setup with a pair of delay lines in place of tape recorders. That's an amusing exercise, but you can accomplish the same thing more easily with a single feedback delay plug-in inserted as a 100-percent-wet send effect. Map a convenient MIDI controller such as a mod wheel to the delay time and set it to range from 1 to 10 ms or so. Feed it white noise and listen to the effect of moving the mod wheel: Increasing the delay sweeps down and decreasing it sweeps up. Feedback is the other critical setting; more feedback yields a more pronounced effect (see Web Clip 1).

Taking Your Time

Using a delay line makes what's going on in flanging transparent. A delay of a few milliseconds shifts the sine wave components that make up a sound (pitched or otherwise) from a fraction of a cycle to several cycles. Some frequencies cancel and others are reinforced in a comb-filter-like pattern in which the combs are harmonically spaced (see Web Clip 2). If you use a sine wave or something similar like an electric piano patch and slowly move the mod wheel, then you'll hear the level rise, indicating reinforcement, and fall, indicating cancellation.

To calculate the lowest cancellation frequency in Hertz, divide the delay time in milliseconds into 500. Even multiples of that frequency are reinforced while odd multiples are canceled. Conversely, to calculate the frequency-cancelling delay time, divide the frequency into 500.

Sweeping the delay time by hand has a number of advantages: You control the range and can vary it with each sweep, you're not limited to a constant speed and, instead of sweeping, you can use the controller to lock in different timbres created by the aforementioned reinforcements and cancellations. But for two-fisted playing, use your DAW's automation options, use a delay with a built-in LFO or use a MIDI LFO output from a soft synth to modulate the delay time. Use an envelope or envelope follower instead of an LFO for a triggered sweep (see Web Clips 3, 4 and 5).

As an alternative, try modulating the delay times in discrete steps, for example with a step sequencer like Propellerhead Reason's Matrix module that sends out control signals. With high feedback, the resonances at the reinforcement frequencies produce an audible pitch sequence.

More Is Less

You can thicken the flanging effect by using a multitap delay or multiple delays in parallel. To retain the sweeping sound, modulate each of the delay times from the same source. You have several alternatives for setting the delay ranges. To best preserve harmonic relationships, use the same ratio of maximum to minimum delay time for each delay. For example, if you set one delay to range from 1 to 2 ms, then you might set others to range from 2 to 4 ms, 3 to 6 ms and so on. Try panning half of the delays hard-left and the other half hard-right. Then swap the maximum and minimum settings for the delays on one side so that one side sweeps up while the other sweeps down (see the figure above and Web Clip 6).

Flanging is typically applied to a single track to produce its signature sweeping sound. For an alternative that is not as pronounced and works well on mixes and submixes, try low feedback and different non-proportional modulation ranges for each tap. That masks the sweeping while adding depth and motion to the sound. Long sweeps of four or eight measures often work better in this case (see Web Clip 7).

Len Sasso is an associate editor of EM. For an earful, visit his Website,swiftkick.com.