[sdiy] Magnetic String Actuation

Tue Sep 13 23:52:09 CEST 2022

I got a random 24V/5kg magnet to try out. It's this one, for reference:
[image: image.png]
It actually works! I tried a few different things it seems like a good
starting point for what I want to achieve.
I first tried driving the magnet with a signal from my VCO, without any
feedback. I didn't even have to tune as precisely as we discussed earlier -
just being in the vicinity and getting the position right resulted in very
obvious sympathetic action. Sweeping the frequency up produced various
harmonics by the string. I didn't get to try more complex sounds yet, but
this is very promising.
I added feedback from the pickups to the magnet, through my eurorack. Sure
enough, there was sustain as long as I got the position right. This is
tricky; moving up and down a string with your left hand means the length is
changing and thus the position you need to be with your right hand to stay
on that specific harmonic. Since the feedback is coming from all the
strings, it's not possible to treat it entirely as an ebow, since moving to
a new string while the old one is still vibrating is messing up your
feedback and gets in the way of the new vibration building up. There is
some sympathetic action here, though, meaning that you can take advantage
of a string that is still sustaining to kickstart the next one. Pretty neat.
It was mentioned that it's not easy to start the vibration from the idle
state, and I found this to be true. I thought, most positive feedback
circuits are kickstarted by their inherent noise, so maybe I should just
add some noise to the mix and see what happens. For some reason, my amp
didn't like it and acted a bit crazy at high levels; eventually I figured
out high pass noise is ok and blasted it through. The effect is like a very
soft bow, and I think it's less dependent on the position you choose.
Mixing some feedback into this seems to help the oscillation start issue,
but maybe it's just my impression. Unfortunately because the pickups also
listen to the magnet's output (it's pretty loud, considering the guitar's
gain) I found limited use for it for now.
I tried impulses as well, which admittedly aren't coming through great in
an AC coupled amp, but still I do get a thump on the string and something
that resembles a very light, rounded pluck. Unfortunatelly the pickup can
catch this too, so the sound is not ideal. Faster looping envelopes also a
sort of sustain , so maybe that's another path to that effect. Turns out
the electromagnet doesn't like this, it got very warm after a few minutes
of thumps coming its way.
All in all I'm very impressed by the results, considering I just bought
random Chinese parts and threw them together in an afternoon. I'd like to
try a few different magnets to get a better high frequency response, but in
the context of a sympathetic string bank this might actually work just
fine. I'll try sending a melody through and see how it reacts to it.
Unfortunately I can't actuate all 7 strings; I'll need to make some wooden
prototype for this to work (and get more magnets and amplifiers). I'm
thinking of picking up the sound (not the feedback) with piezo pickups; do
I need an actual acoustic chamber for this to work or is a simple wooden
plank fine, to get things going?
I'll upload some samples later. This has been in my mind for 10 years give
or take and I can finally see a path to making it work thanks to everyone
that chimed in!

On Tue, 13 Sept 2022 at 12:25, ulfur hansson via Synth-diy <
synth-diy at synth-diy.org> wrote:

> you can also run current (signal) through the string itself - and place
> magnets close to thw string - be careful not to zap yourself though by
> accidentally touching the strings, also make sure the string gauge is heavy
> enough to handle the current :)
>
> sent from outer space
>
> > On 13 Sep 2022, at 09:06, René Schmitz <synth at schmitzbits.de> wrote:
> >
> > On 13.09.2022 08:26, Spiros Makris via Synth-diy wrote:
> >> Alright! Then it sounds like I'm on the right track since I got the
> cheapest electromagnets (2.5kg) I could find on aliexpress! I'm kinda
> itching to give this a go so I'll pay a visit to my local robotics shop and
> get another cheap one and give it a go sooner.
> >>
> >> Harry that's useful, I'll try the middle of the string length for my
> next experiment. While you are right, I do think my tuning was good enough
> to warrant some sympathetic action. It's true that the sustainer
> configuration will probably produce vibration more easily, since it cuts
> out any problems with tuning.
> >
> > I'm thinking, you should try to drive it at half the frequency of the
> natural frequency of the string, as with a simple plain electromagnet, and
> an non-magnetized string, the force is attractive regardless of the
> polarity of the current.
> >
> > So you get two peaks of force per cycle when you drive it with a sine
> wave.
> >
> > This is a <electroboom> full bridge rectifier </electroboom> action,
> which doubles the frequency.
> >
> >
> >> My goal is to create a sympathetic string bank and excite it with sound
> from synthesizers or other electric instruments (ie guitars, lutes etc).
> This effect is most common on instruments like the sitar, but all stringed
> instruments have some sympathetic effect going between the strings while
> you play them
> >>  I expect that by adding the right amount of positive feedback I can
> get a sort of sustain control, and self-oscillation when cranked up. My
> assumption here is that the actuator, string, amplifier and pickup
> essentially form an oscillator, with the string acting as a high Q
> resonator, akin to an LC tank.
> >
> >
> > An Ebow has a magnetic bias in the form of a pre-magnetized core (both
> in the pickup and the transducer),
> >
> > so that they can use the same frequency that they pick up. You might
> want to add a DC current to your magnet to simulate this, and be able to
> use feedback. Or stick a permanent magnet on your core.
> >
> >
> > Best,
> >
> >  René
> >
> >
> > --
> > synth at schmitzbits.de
> > http://schmitzbits.de
> >
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