How exciting! Adding mechanical feedback to a guitar.

[reTrEaD]

At the moment there is a bit of discussion on the forum about exciters. Speaker-like devices that are intended to "turn any surface into a speaker". Devices like this:


www.theloudspeakerkit.com/dayton-audio-daex19ct-4-coin-type-19mm-exciter-5w

They are available in various sizes and mounting styles.  Most common is a peel and stick adhesive.
Power requirements range from half-watt to tens of watts.  Most common are three to five watt max.

Some thoughts have been expressed for using these to add mechanical feedback to a guitar.

Some of the hurdles that need to be dealt with:

Where to mount it?
Will the adhesive cause permanent damage to the guitar finish if we mount it externally?
How to amplify and CONTROL the signal driving it?
If we try to hide the exciter and its amplifier inside a guitar cavity, how do we get power to the amplifier?
Will electromagnetic feedback get back to the guitar pickups and wreak havoc?

Because there is a LOT to discuss and the early discussions are derailing another thread, I chose to create this one.
This would be a good place for general questions.
Maybe we can figure out some things before someone takes a bad turn.
I have no personal experience here, so anyone who has is certainly welcome to take the lead.
Else we can all toss in our views whether from experience or speculation.
Links to existing projects on other sites are welcome.
And of course, if anyone chooses to start a thread on their own project and what they've learned, that would be good too.

[haydukej]

I would venture to say that Ash has probably done the most, or at least documented, with these on here. Check out this thread and the one linked internally.

guitarnuts2.proboards.com/thread/7089/wanna-hear-guitars-feeding-back

[JohnH]

This could be very interesting on an acoustic guitar, to control, suppress or enhance the natural resonances.

At one extreme, piezo sensors (used for vibration sensing but not for tone output) could be placed close to transducers, in a negative feedback mode to cancel body resonance (and hence acoustic feedback) at key points of max vibration amplitude. Or, they could enhance and control, to make a modern dreadnought sound like a smaller parlour guitar, or change the signature to that of Grandpa's pre-war D-28.

[ashcatlt]

Dec 22, 2016 12:02:18 GMT -5 reTrEaD said:

Where to mount it?

About anywhere on the body works fine. I had some results with it on the headstock, also, but that's kind of uncomfortable having a cable hanging off the wrong end of the axe, and really just sticking it somewhere between the bridge and the strap button works on most guitars.

BUT it does spit out a whole bunch of EM radiation which will be induced into any nearby wiring or pickup, so unless you're looking for something like the high-pitched squeal of a microphonic pickup, probably keep it away from those things. That includes the bridge itself! Foil shielding doesn't help.

Will the adhesive cause permanent damage to the guitar finish if we mount it externally?

Depends what adhesive. The sticky foam tape that came with mine came off without much trouble. A little rubbing and lemon oil and it's gone. I've been using double-sided carpet tape which is a bit more serious, but still cleans up okay. I'm sure different finishes will respond differently to different adhesives, but I haven't had any real problems. (Course, I have duct taped my GK3A to my SG clone, so might not be the best to ask...)

How to amplify and CONTROL the signal driving it?

What I did was wire it to one end of a cable with a 1/4" TS plug on the other end and I drive it from a headphone amplifier. I have found that a hefty dose of compression or just plain distortion helps a lot. I'd imagine driving it from a heftier amp might reduce that necessity some.

Will electromagnetic feedback get back to the guitar pickups and wreak havoc?

Yep! I talked about that above. One thing I didn't expect was the coupling between the cables. Even with the driver itself well away from any wiring, if its cable gets close to the guitar cable, you'll hear it. Didn't cause squealing in my world, but there were times I wanted a relatively clean guitar sound, but still had to distort the driver signal some, and I had to be really careful to keep that fizzy distortion from being noticeable in the finished product.

Because there is a LOT to discuss and the early discussions are derailing another thread, I chose to create this one.

Where is this thread? My thread that haydukej referenced was almost as much spam as anything else ( ), and I have since gone back to just pounding through something loud when I want feedback in the studio, but I've still got them around and can test things if anybody has anything more they want to know.


Edit - and of course JohnH just goes and takes it miles past anything that ever occurred to me. I just wanted feedback.

[reTrEaD]

Dec 22, 2016 19:19:23 GMT -5 ashcatlt said:

Where is this thread?

It's one of antigua's threads in the Pickup Testing and Modeling subforum.
It took a hard left turn after I replied to this post:
Modeling an electric guitar with LTSpice
I thought it better to make a new space for this tangent and let that thread serve its original intent.


You've added a lot of value to the conversation with your real-world experience. Particularly this part:

Dec 22, 2016 19:19:23 GMT -5 ashcatlt said:

Dec 22, 2016 12:02:18 GMT -5 reTrEaD said:

Where to mount it?

About anywhere on the body works fine. I had some results with it on the headstock, also, but that's kind of uncomfortable having a cable hanging off the wrong end of the axe, and really just sticking it somewhere between the bridge and the strap button works on most guitars.

BUT it does spit out a whole bunch of EM radiation which will be induced into any nearby wiring or pickup, so unless you're looking for something like the high-pitched squeal of a microphonic pickup, probably keep it away from those things. That includes the bridge itself! Foil shielding doesn't help.

This confirms one of my suspicions.

[blademaster2]

Initially I was thinking of a piezoelectric speaker element, so magnetic feedback would not be as likely from it. A voicecoil will, of course, need to be kept away from the pickups to eliminate direct feedback and squeal.

For the cable feedback problem, I would try twisting (even twisting and shielding) the feed/return of the speak signal to eliminate/reduce EMI emissions. If that was already tried and it did not work then it is a more difficult problem and might need more separation and/or differing orientation (if directionality is pronounced on the pickup).

[reTrEaD]

A piezo element would be a great way to avoid the EM issue but could you find one that could deliver enough mechanical movement to get the job done?

[blademaster2]

That is a good question. I am afraid that I do not know very much about them, except that Piezo are stiffer but might require very different drive signal from an audio amplifier expecting to drive an 8 ohm speaker. Their displacement would be tiny, but if stiff enough in its movement it may be a good excitation to guitar body material.