That awful noise.

October 4, 2007 12:42 PM

How do I tackle feedback with a piezo pickup on a very small instrument (ukulele), running it through guitar amps or PA systems?

So I have a concert ukulele, and I've hooked it up with a K&K Big Shot piezo pickup, positioned right behind the bridge. I've been running it through either a Fender Princeton 112+ or Peavey Classic 30 guitar amp, and I've found that any high volume will cause almost unavoidable feedback - even when I'm holding all of the strings still. The problem seems to be reduced when I cut off the mid & bass at the amp level. I know that distance is pretty key here - I need to be further from the amp - but it's still pretty bad. Is there anything that performers do about this that I don't know about? I'd really like to get this thing in action and on stage someday.

Thanks!

posted by tmcw to media & arts (12 comments total)

Have you tried putting the amp in front of you and standing either a bit left or right of it?
posted by drezdn at 12:49 PM on October 4, 2007

It's been a while since I've dealt with this sort of thing, but first keep in mind what feedback is. The pickup is creating an infinite loop when it re-picks up its amplified signal, then the amp amplifies it again, etc.

It's worse with acoustic instruments because they tend to resonate with the sound coming out of the amp.

One way you might be able to solve it is by using a preamp with an EQ, and rolling off the frequencies you mentioned before.

Another might just be amp placement. There are places you can stand in relation to the speakers on most amps that will lead to more or less feedback, you can also try redirecting the amp (say put it on a chair angled towards your head).

If your ukulele has F holes, you might want to try covering them with tape. I'm not sure if it actually works, I've just seen it done.

Just try playing around with it.
posted by drezdn at 1:00 PM on October 4, 2007

You should be able to avoid a lot of feedback by standing with your back to the speaker. I have a similar setup for my banjo and I've found that that's the simplest way to cut down on the feedback. Te best way, I've found is to sit on top of the amplifier, but that has certain drawbacks, like lack of monitoring.

Also, if you're trying to kill a feedback loop, try muting the strings and putting your palm to the uke top - it will vibrate too.
posted by lekvar at 1:08 PM on October 4, 2007

Feedback Ferret!
posted by cosmac at 1:27 PM on October 4, 2007

Depending on what kind of bridge it has, you might have better luck with an under-saddle pickup.
posted by leapfrog at 2:12 PM on October 4, 2007

stuff the body full of some bunched up polyurethane foam.

it will kill the acoustic volume, but it's easy enough to take out later, does not harm or alter your instrument, and should help a LOT. with a uke, the piezo sound you're getting won't be altered too dramatically (certainly less so than if you go the EQ route).
posted by quarterframer at 2:32 PM on October 4, 2007

Do you have a preamp? I've recently been experimenting with a piezo pickup on my acoustic fiddle, and a preamp has really improved things. (It also generally helps with tone and volume control. I use a Fishman Pro-Eq II - not anywhere near the top of the line, but it definitely makes a difference.)

Covering f-holes (if applicable) and experimenting with where you position the amp may also help. You may also want to check out other kinds of pickups - for fiddles, at least, bridge pickups, contact pickups, and other varieties have different advantages and disadvantages (quality, price, etc.)
posted by ubersturm at 3:51 PM on October 4, 2007

The two things I would try first would be:

a noise gate, which is an effect that sits in your signal path and cuts off the signal below a certain threshold. It can prevent feedback from developing.

padding, buffering or shimming the pickup mount where it contacts the body, with something vibration-absorbing, like rubber or spongy foam.

If those didn't work then yeah, stuff the body with cotton wadding or foam. You have to really cram it in there; the purpose is that it absorb the vibrations of the wooden top and back.
posted by ikkyu2 at 4:35 PM on October 4, 2007

Foam it up. You need to change the resonant characteristics of the body.
posted by Wolof at 5:58 PM on October 4, 2007

Disclaimer: I'm not a sound engineer, (regular) musician, or audiophile.

I used copper foil shielding tape to line the cavity behind the pickups of my electric bass and cut out undesired hum and feedback due to (what I assume is) house wiring and/or ambient interference.

Would a layer of copper foil shielding tape inside the uke beneath the bridge make a difference, I wonder?
posted by bonobo at 11:17 PM on October 4, 2007

You want a graphic equaliser like this or this.
posted by oh pollo! at 1:11 AM on October 5, 2007

I think your answer is in your K&K Big Shot link:
This top performance, single-head piezo transducer was specially designed for instruments with low mechanical resonance. These include instruments made of relatively thick material, such as the sound board of a piano. It is considerably harder to induce vibration on these thicker materials compared to the thin top of a violin, for example.

It sounds like you need a different pickup, the one you're using is not designed for a highly resonant instrument like a uke. See if you can take it back.
posted by InfidelZombie at 9:07 AM on October 5, 2007