running a diy "motor" off audio signal

Thread Starter

noisepunk

Joined Feb 11, 2022
2
hi all,

new here, first post.

i'm working on an art installation involving a mechanically rattled window and recorded/played-back sound, and thought it might be interesting and closer to the intent of the piece to have the rattling mechanism actually controlled by audio signal so the rattle and sound are directly in sync. i have already gotten the window to rattle with a medium sized vibration motor (DC motor with an eccentric weight on the shaft), so i know that part works.

i designed a simple mechanism using an electromagnetic coil (haven't built it yet, just a drawing), which would replace the DC motor, and my thought was that in theory i could wire this in place of, or in parallel or series with a subwoofer so long as i wound the coil in my "motor" to the correct impedance.

i know that on a very basic level a speaker and motor work on the same principal, and that there must be some way to make this work, but i strongly suspect i'm oversimplifying things and missing something crucial as i've only a cursory theoretical understanding of how all of this works. i've also hit a wall on what to research in order to answer these questions myself, or i wouldn't be asking here. i'm not really trained in anything electrical or electronic, all of my knowledge and experience has just come from simple hobby circuits, and a bit of work with electric guitars and amplifiers.

hope all of that is fairly clear– here's the diagram for the mechanism i'm imagining (the dark grey bit would be a weight of some sort–i haven't worked out how heavy it should be, or any other dimensions for that matter), which would be mounted to the side of this custom window frame that's intentionally been left a bit loose in the wall it's mounted to:

coil shaker motor.png

-thanks!
 
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bassbindevil

Joined Jan 23, 2014
422
The "voice coil" head positioning mechanisms from big old hard drives might work.
Apart from the size, a DIY "bass shaker" could be ideal. The most common method is to modify a donor woofer (one with a rotten surround is perfect).
 

Thread Starter

noisepunk

Joined Feb 11, 2022
2
a "bass shaker" (i didn't know they had a name!) was my first thought, but i wonder if it's the most kinneticly efficient way of doing this–i.e. i wonder if i couldn't get more shake with less amplifier power out of something that's meant to physically move a bit more than a speaker voice coil is.

i'll definitely consider that a bit deeper though. the window is going in a false wall, so i could pretty easilly conceal a fairly large woofer in there so long is it wasn't deeper than the width of the window frame.
 

Tonyr1084

Joined Sep 24, 2015
6,898
I recall tearing down a sonic toothbrush. The motor was rather curious as it consisted of - um - the motor, but the shaft was captured and limited by a torsion bar so that it could only rotate just a few degrees, if that much. The electronics - I'm guessing - attempted to drive the motor in one direction then rapidly attempted to drive it in the opposite direction resulting in moving the toothbrush head slightly in each direction.

I don't know how much vibration you want on the window but I'm imagining that such a motor with a small anvil on the head could rap on the window frame at very high rates, depending on the AC frequency you try to drive it. It's been a while since having pulled that thing apart and I don't know where the guts got off to so I can't grab it and photograph it to show you what I'm thinking of. But possibly even a regular DC motor driven from an AC source with the shaft motion restricted by some mechanical spring system might accomplish something similar. Driving it with an audio source might work. However, I'm sure there would be a particular frequency where it would resonate while other frequencies would be subdued. Harmonic frequencies may respond better. One would have to experiment to see what works best.

Sorry I don't have a more specific answer to your question, I'm just throwing this out there and maybe someone else can pick it up and run with it.
 

bassbindevil

Joined Jan 23, 2014
422
Voice coils have to be quite powerful to accelerate even a lightweight cone at relatively high frequencies. I can't remember the math; try this video:
Maybe a low-inertia servomotor would be more efficient, but it would also cost much more (about $100 and up for a used one that can work at audio frequencies), and they have coil impedances too low for typical audio amplifiers.

I made a bass shaker for well under $20; 8" Philips woofer (1" voice coil) from a $10 pair of thrift store speakers with rotten surrounds, spring made from a strip of 1/8" thick printed circuit board, 1/4" threaded rod to a T-nut epoxied into a plastic pipe cap which was epoxied around the woofer dust cap. It seemed adequate to rattle a window, but if you have any doubts, a used car sub should be much more capable. Vintage car swap meets often have people selling old car audio stuff, or there's pawn shops, yard sales, thrift stores, auto wreckers. A perfect substitute for the 1/8" PC board stock could be a sailboat batten, since those are also made from 1/8" fiberglass, and under $10 should get you enough to build two bass shakers.
This is similar to the one I made, but with a much beefier woofer.
 
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