[sdiy] Magnetic String Actuation

Mike Bryant mbryant at futurehorizons.com
Tue Sep 13 10:24:48 CEST 2022

Well worth trying driving at the centre, and then at the 1/3, 1/4, 1/5, 1/8 and so on points to see if you can stimulate more harmonics.

Another thing that may work is to square up the feedback and drive it near one of the ends.  The string will turn this energy into its resonances in a similar manner to plucking, and you would be building a mageto-mechanical version of a Karplus-Strong filter which can be used to simulate plucked strings of course.

From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of Spiros Makris via Synth-diy
Sent: 13 September 2022 07:27
To: Harry Bissell
Cc: synth-diy at synth-diy.org
Subject: Re: [sdiy] Magnetic String Actuation

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.
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.

On Mon, 12 Sept 2022 at 23:08, Harry Bissell <hbissell at wowway.com<mailto:hbissell at wowway.com>> wrote:
Something to think about is the string itself. It is a very “hi Q” resonator. If tuned to 440hz you probably need to be very close to that or you will get no response. You can excite harmonics as well, but if you locate your driver at an anti-node you will get nothing. For fundamental the best drive position is halfway down the string. At the bridge is very inefficient but you’d get the best frequency response (number of harmonics you could drive. You might want to consider a horseshoe magnet which would pull the string over a wider area.

Is the idea to excite the string with a similar frequency, or maybe just a pulse and let it ring….


Sent from my iPhone

> On Sep 12, 2022, at 2:47 PM, cheater cheater via Synth-diy <synth-diy at synth-diy.org<mailto:synth-diy at synth-diy.org>> wrote:
> he did say he'd just try buying the cheapest thing from china first
> because those actually worked just as well.
>> On Mon, Sep 12, 2022 at 8:38 PM cheater cheater
>> <cheater00social at gmail.com<mailto:cheater00social at gmail.com>> wrote:
>> he's not a particularly organized fella, he probably just had some of
>> these and some of those. maybe he couldn't get enough or whatever.
>>> On Mon, Sep 12, 2022 at 8:24 PM Mike Bryant <mbryant at futurehorizons.com<mailto:mbryant at futurehorizons.com>> wrote:
>>> Looks like those are standard industrial electromagnets with heatsinks stuck on top of them as they are being overdriven.  Not sure why some have larger heatsinks.
>>> Possibly something like these https://uk.farnell.com/stephenson-gobin/58-0130-24-vdc/electromagnet-type-58/dp/7200869
>>> -----Original Message-----
>>> From: cheater cheater [mailto:cheater00social at gmail.com<mailto:cheater00social at gmail.com>]
>>> Sent: 12 September 2022 19:07
>>> To: Mike Bryant
>>> Cc: Spiros Makris; Andrei Kudryavtsev; synth-diy at synth-diy.org<mailto:synth-diy at synth-diy.org>
>>> Subject: Re: [sdiy] Magnetic String Actuation
>>> here you go, that's made by a guy i know https://www.spectrumpiano.com/
>>> he hired an EE to do the electronics.
>>> he says that pretty much any electromagnet that will fit the space will be fine. however, he's kind of an ass so he doesn't want to say what sort of electromagnets he used himself. i guess he wants to be the king of electromagnet piano or whatever, sheesh.
>>> On Mon, Sep 12, 2022 at 12:41 PM Mike Bryant <mbryant at futurehorizons.com<mailto:mbryant at futurehorizons.com>> wrote:
>>>> No idea if this would work or not, but you could try removing the frame, cone and all other moving parts from an old 25W-ish loudspeaker and see if you can make a string resonate using it as at least you know you can drive it using any amplifier.
>>>> Even by taking an old guitar string and stretching it inside the speaker coil possibly ?

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