[sdiy] Magnetic String Actuation

Spiros Makris spirosmakris92 at gmail.com
Mon Sep 12 11:49:32 CEST 2022

Thanks for all the resources and advice! Andrei, I the MRP is what set me
down this path years ago, and even though I've gone through the papers (and
did so again in the weekend, after your suggestion), It's still not enough
for me to figure out which off the shelf component might be a reasonable
starting point. For example, the size, resistance and inductance of the
electromagnet is mentioned, along with a vague power rating ("20W for a
brief time" or something along those lines, but unfortunately listings
don't mention any of those specs (usually just wattage and pulling force),
so it's still vague for me. I'm trying to move strings with much lower
tension involved (basically, an electric guitar of sorts), but any details
of your setup that you might be able to share will be really helpful.
I didn't get to do any new experiments, but I ordered a few electromagnets
from China, hoping I'm lucky and they are at least somewhat suitable. It's
going to be a while before they arrive, so I'll have to fool around with
other parts/ideas until they do.
About the experiment I run so far, with the pickup I mentioned in my
previous email:
The coil was placed at various spots along the string so I expect I
eventually must have placed it over the correct position. I tuned an
oscillator to the frequency of the bass string of my instrument and tried
to drive the signal into the coil using a class D amp. I could hear the
coil vibrate and whenever I pointed it towards the pickups of the guitar,
they would pickup the sound from more than 10cm away. Moving the actuator
in the vicinity of the pickups' permanent magnets, it vibrated, essentially
acting as a speaker coil, which is to be expected I suppose. I tried
various placements along the string, with different orientations and power
levels but couldn't get anything from the string. I *think* I managed to
get some very slight vibration eventually but it was so faint I'm kinda
doubting it now, in retrospect. I also tried to lightly pluck the string in
order to get the vibration started, with no obvious effects. This was
repeated with a permanent magnet placed on the coil, which seemed to change
how the coil interacted with the pickups but other than that didn't improve
I wanted to try the sustainer configuration as well, but didn't have any
preamps handy in order to patch it up. Maybe I can dig into my boxes and
give it a try today.
I'm starting to realise that the electric guitar is less than suitable for
these experiments. The pickups listen to all the EM rubbish I'm throwing in
the air making any sort of feedback that involves them very impractical,
not to mention the interaction between coil and pickup magnets. Maybe I
should tension a steel string or two on a wooden plank and add a piezo
pickup to get over this limitation.
I'll try to get on #audioworld and see if I can track down efex. Hopefully
they'll be inclined to share some of their wisdom on the matter.


On Thu, Sep 8, 2022, 23:39 Andrei Kudryavtsev via Synth-diy <
synth-diy at synth-diy.org> wrote:

> Hi,
> I suggest studying all papers from the research on MRP that Andrew
> McPherson ran over a decade ago. There are several, not one or two... It's
> exactly what you're trying to achieve and very easy to demonstrate how
> difficult the task is to induce the string vibration when it is at rest.
> That's why MRP works and sounds that way.
> All research is public including hardware decisions.
> I'm in MRP topic quite deeply, already have it ongoing for quite a while
> with a donor piano already. Magnetic resonators are off the shelf.
> http://instrumentslab.org/research/mrp.html
> On Thu, Sep 8, 2022 at 12:46 PM Olav Martin Kvern via Synth-diy <
> synth-diy at synth-diy.org> wrote:
>> In addition to the amazing Segulharpa mentioned earlier, you might find
>> this instrument from Godfried-Willem Raes/Logos Foundation interesting
>> (note: there is some minor "artistic" nudity on some Logos Foundation
>> pages):
>> https://logosfoundation.org/instrum_gwr/aeio.html
>> Thanks,
>> Olav
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