Overheating Coils with Audio Signal

Hello Forum,

Warning: Incoming Wall of text, I would like to be brief but I've noticed in other forums that my project is not understood well enough so I'll try to explain it with more detail in order to obtain more adequate answers.

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I'm a new media artist and I'm currently working on a project which involves strings in motion, these strings are oscillated with the following device: https://www.pasco.com/prodCatalog/WA/WA-9857_string-vibrator/ and the setup looks something like this: https://www.youtube.com/watch?v=BSIw5SgUirg

Inside the device consists of a copper coil, a rare earth magnet and an aluminium reed, I already made my home imitation: **broken link removed**

So far so good. Now the problem:

I need to control the frequency, but the "Sine wave generator" or any other generator wouldnt work for me for the following reasons:

- I need to automate the process
- I need to control several strings simultaneously
- I need sometimes to overlap more than one wave in a signal

It occured to me the best way to adress all these things at once would be with audio signals, but when I plug the device to a line output it won't gain enough amplitude, so I decided to amplify. Once amplified, the string will move as intended, but in no time the coil overheats, and this is my headache, I don't know how to prevent overheat form happening without losing amplitude.

It is important to note that this project would be exposed for several hours at galleries so time of use is an important factor to take into account.

I would like to hear any suggestions or ideas, I could even hire someone to help me solve this.

Thanks a lot in advance!

Offhand one way to maintain the same amplitude with lower current would be to wind more turns of smaller wire on the coil and perhaps a stronger magnet. Is it you homemade version that has the problem? Is that long piece of metal in the picture aluminum?

Although you are using a strong magnet, the homemade actor system is probably rather ineffective, turning most of the input power into heat. A voicecoil (dynamic speaker magnet system) would be more effective.

Secondly an audio signal contains many spectral components that are not exciting string resonances but generate unwanted heat.

Audio may still work, but the problem should be considered.

The original vibrator works only with one continuous tone of 50Hz to 60Hz. Audio rarely produces these frequencies and produces many other frequencies, and produces many frequencies at the same time. Also audio does not produce a tone of from 50Hz to 60Hz continuously.

In order to see the waves on the string then the tone frequency, length of the string and the tension of the string must all be adjusted to match each other. Since audio has various frequencies and levels then audio cannot be used to activate the device.

Maybe you could have a musician design a piece of music and the sub-woofer amplifier output can produce a few continuous tones from 50hz to 60Hz that match the length and tension of a string.

Hello again, thank you so much for your answers!

Now to respond:

@crutschow:
- Its been hard to find someone able to help me building my coils, I hired an electrician, he seems to be good and he already made one for me with 4 smaller copper wires and adjusting the size so its the closest to the magnet, it still overheats.

- I'm using a neodymium magnet similar to these https://www.rare-earth-magnets.com/c-8-cylinder-magnets.aspx , I don't know if there are any other stronger magnets available commercially, any hints are welcome, this is not my area of expertise.

- Yes, it is aluminum.


@FvM: I also got this device https://www.pasco.com/prodCatalog/WA/WA-9854_economy-wave-driver/ , which works with a voicecoil, it works very good at vibrating the string and actually most of my experimenting was done with such device. My problem with it is that it sounds much louder than the "string vibrator" and my project requires as less sound as possible, it is more concerned on the visual aspects of the string in motion rather than sound, aesthetical whims...

@Audioguru and FvM: My signal is not any audio signal, I am programming singular sine waves with Max/MSP, unless the audio output itself introduced overtones that I'm not aware of, I'm pretty confident my original signal is stable and free of any harmonics.

Regarding the design on the piece, I'm actually a composer (this is why I am so lost here) and the goal is (more or less) to compose a piece to be seen rather than heard .

Audible sound will be generated by any object vibrating with respective frequency, according to it's surface and oscillation magnitude. The oscillating string itself will probably generate most of the total sound power.

I don't see the point why a moving coil actor should generate more unwanted sound than a moving magnet design, equal oscillation amplitude provided. I fear the main difference is by the larger amplitude generated by the voice coil. The moving magnet has a larger accelerated mass and is feeding more structural sound into the support.

A technical moving coil system may have unwanted radiating surfaces, e.g. the coil suspension. It's a matter of suitable design to minimize unwanted actor sound generation leaving only the sounding string. I can imagine that the said commercial device isn't designed to minimize actor radiated sound, but as said, it may be an illusion that the string will be silent with a different actor.

The misleading point in your initial post was the term "audio signal" where you actually meaned "audio amplifier".

What is the size of your coil? The magnets you linked to are tiny and must have a close fit inside the coil so maybe your coil is also tiny?
What is the resistance of your coil?
What is the RMS output power of your amplifier (rated into an impedance a little higher than the resistance of your coil)?

The speaker used as a vibrator is a speaker with a cone that produces sounds so of course it will be heard. The cone suspension keeps the voice coil from rubbing on the magnet. How do you suspend your magnet so it does not rub on the coil? It must have a fairly small gap between the magnet and the coil for a good driving force.