What is the name of this audio effect?

May 2, 2005 6:13 PM

I'm looking for a device that processes an audio signal by sweeping the center frequency of a bandpass filter back and forth across a frequency range. What is this effect called, and what sort of effects processor might provide it?

This seems like a really simple idea, so I'm sure somebody must have tried it, but I don't know what such a gadget might be called. I've found schematics for bandpass filters and for LFOs, and a project that combines the two is probably (barely) within the range of my electronics ability, but it would take a lot of time and I'd rather just buy the gadget if it exists. I'd like to be able to set the low and high ends of the frequency range, and it should be possible to set the sweep rate to at least around 5sec/cycle. The perfect solution would be a simple guitar stompbox type device, since I'd like to run this on battery power.

posted by Mars Saxman to Technology (8 answers total)

That sounds like an Autowah or envelop filter to me. I haven't any firsthand experience, but the site I've linked to seems to have a bunch of information. And you're in luck, it's a very common stompbox, esp. for seventies funk bass.
posted by stet at 6:22 PM on May 2, 2005

That's what a wahwah does.
posted by ludwig_van at 6:40 PM on May 2, 2005

Your best bet, Mars, is to go to a guitar store and audition the following: phasers, flangers, and chorus. All models sweep both up and down and offer a rate adjustment. Some models allow you to set the upper and lower bounds.

If you want to do the sweep manually, check out the wah-wah pedals.

Auto-wahs and envelope filters have the added benefits of automatically changing a parameter in proportion to the attack strength. They also give you a choice of sweeping up or sweeping down. Some give you the to sweep in both directions. The latter are the most expensive.
posted by mischief at 6:43 PM on May 2, 2005

Wahs don't work automatically. They require user input to cycle through the freq range. This input is taken through a foot pedal with the initial concept being a tone knob worked with the foot instead of the hand. An autowah/envelope filter does the same thing, but the modulation is controlled automatically instead of by the user. The relationship is the same as a built-in amp tremelo modulating the volume at a constant rate versus pumping a volume pedal to achieve the same effect.

(Also, what mischief said.)
posted by stet at 6:49 PM on May 2, 2005

Best answer: You were pretty close Mars..this effect is called a "filter sweep". More specifically it sounds like you're looking for a cyclic filter sweep. On a synth(analog/modeling/virtual) this would be created by locking an LFO to the filter.

Google: "filter sweep"+pedal
posted by mnology at 10:21 AM on May 3, 2005

An Auto wah is a filter sweep controlled by an LFO.

An Envelope filter is a filter sweep controlled by input amplitude.

A Wah Pedal is a filter sweep controlled by foot.
posted by sourwookie at 11:25 AM on May 3, 2005

Might also be a "flanger."
posted by jpburns at 1:05 PM on May 3, 2005

You want a Moog MuRF. Flanger no - delay-based. Phaser no - phase based. Wah no - it works by varying capacitance, not filter parameters. Envelope filter yes - the MuRF is the acknowledged champion of the breed, smashing all its foes to the ground.

Buy me one too - I MUST HAVE ITS POWER!!!

uh, sorry.
posted by ikkyu2 at 5:53 PM on May 3, 2005