[sdiy] Magnetic String Actuation

[rburnett at richieburnett.co.uk]

A few hundred turns of wire around an iron nail result in the magnetic 
field being spread over quite a large volume.  The air gap is large, so 
the field lines leave one end of the nail and travel quite far out into 
all the surrounding air before finally entering back into the other end 
of the nail.  As a consequence, the field strength is quite weak.

If instead you form the magnetically permeable material into a 
"horseshoe" shape with the copper wire wrapped around it, you will get a 
much more concentrated magnetic field in the air gap between the two 
ends of the "horseshoe".  This is how things like tape heads or motors 
work.  You want to keep the field lines confined to a small air gap.  
Something like a MnZn ferrite toroid with a piece cut out might work 
well with the magnetic string placed in the air gap.

Also, as others have said, there are nodes (where the string doesn't 
move) and anti-nodes (where the movement is maximum)...

https://www.physicsclassroom.com/class/waves/Lesson-4/Nodes-and-Anti-nodes

...so attempting to drive the string into resonance by forced CW 
excitation will only be effective if applied at the correct position.

-Richie,



On 2022-09-08 18:04, Quincas Moreira via Synth-diy wrote:
> I'm not sure about it but I think the ebow picks up the string
> vibration and amplifies it back into the coil, ensuring the
> sympathetic resonance due to the direct feedback.
> 
> On Thu, Sep 8, 2022 at 10:03 AM Mike Bryant
> <mbryant at futurehorizons.com> wrote:
> 
>> Where are you positioning and setting the angle of your
>> electromagnet with regard to the strings ?
>> 
>> For example pushing large amounts of energy into an anti-node is
>> probably not going to do anything.
>> 
>> From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf
>> Of Spiros Makris via Synth-diy
>> Sent: 08 September 2022 15:02
>> To: brianw
>> Cc: synth-diy mailing list
>> Subject: Re: [sdiy] Magnetic String Actuation
>> 
>> thanks for all your suggestions! I started writing a response to
>> everyone in the morning, but eventually got some supplies before
>> hitting send, so scrap that.
>> 
>> So I went on and got a spool of 0.3mm diameter enamel wire and went
>> on to make an electromagnet with about 500 turns on around a metal
>> screw.
>> 
>> Came out 8ohms and 2.2mH. My amplifier module can drive up to 24W
>> into this little thing but starts cutting off after maybe 1-2kHz.
>> Turns out 0.3mm is pretty thin, anything above 0.5W makes it warm
>> up. I even got some fumes out of it!
>> 
>> I can hear the coil vibrate and my instrument's pickups listen to
>> whatever the coil is putting out (it's pretty loud if you point it
>> towards the pickup), but I can hardly get any sympathetic action. I
>> *think* I got a very slight vibration after fiddling around with it,
>> but I'm definitely not even close yet.
>> 
>> I'm not sure what I should try next? Maybe I need more turns
>> (inductance) to produce a stronger force? Could the coils' frequency
>> response be way off and that's causing problems?