Comb Filter

Comb filtering occurs when you delay a signal by a small amount and recombine it with the original signal. This results in cancellation of certain frequencies and creates a distinctive, thinned-out sound.

Adding a comb filter to one of your tracks is easy:

1. Solo the track you’re working on so you’ll be able to hear the effect.
2. Add an AUDelay in one of the effects pop-up menus.
3. Click the Effect Edit button to open the AUDelay parameters.
4. Set the Dry/Wet Mix to 50%, the Feedback to 0 (the middle value on the slider), and the Lowpass Cutoff to 22050 Hz (the maximum).
5. Since you’re working with extremely small values, you’ll have to set the Delay Time using the text box to the right of the slider. Start with 0.002 Seconds.
6. Hit the Play button and toggle the effect on and off. You can either click the On checkbox in the AUDelay dialog, or click the checkbox next to the effect pop-up in the Track Info dialog. You should be able to hear some frequencies drop out when the effect is on.
7. Play with the Delay Time settings until you get a sound you like. For comb filtering, anything between 0.0001 and about 0.0099 should work (see Figure 3). Different times will affect different frequencies, so feel free to experiment.
8. For an even stronger effect, create a second AUDelay in the other pop-up menu. Use the same settings to double the strength of the comb filter, or try using two different delay times. If the delay times are close but not exactly the same you can get some strange ringing effects.

Figure 3: Settings for the comb filter. Delay times between 0.0001 seconds and 0.0099 seconds will work.

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About the Author

Jeff Tolbert is a musician, painter, and graphic designer living in Seattle. He plays bass and guitar and is becoming passable at keyboards. He has played in numerous bands over the years, including What Fell?, the Goat-Footed Senators, the diary of Anne Frank String Quartet, 80 Bones, and the Fireproof Beauties. When GarageBand came out, Jeff got so excited about making music on his iBook that he went out and spent close to $1000 on music equipment and software. He now routinely stays up until the wee hours creating songs and textures to amuse and irritate himself and his friends (http://www.jefftolbert.com/).

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