[sdiy] Magnetic String Actuation

[René Schmitz]

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


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