Nephew's science project (balloon speaker)

[Torben]

Hi all,

Preface: this is a bit of a MacGyver problem. I have only a little information on it. I'm over 1000 km from the problem, the people giving me information on the problem know far less even than I do about electronics (what's a resistor?), and they have basically no resources available to them to help solve it. Anybody up for some jerry-rigging?

OK. So I got a call from my kid sister today. Her son decided to make a balloon speaker for his science project. For those who haven't heard of this (I hadn't until tonight, although I know of similar ideas), you can see the video he used as a basis here: How to Make a Speaker from a Balloon - Video

So my nephew took a length of speaker wire, ripped it in half lengthwise, and made a 25-turn coil about 2.5" in diameter. Stuck to the side of a balloon and plugged into a stereo, and with a magnet placed inside the coil, they heard sound. It got him through the local science fair and now he's going to the district science fair.

So far so good. The problem is that they've killed 1, maybe 2 out of the 3 stereos they've used it with. It works for a while, then becomes inaudible. One of the amplifiers worked with its original speakers afterward, and another amp didn't. The third one they're not sure.

My suspicion is that the impedance is too low for the amplifiers and the amp output stages are dying from trying to feed what is almost a short circuit. I expect they're rated for either 4Ω or 8Ω speakers but I doubt this thing is presenting that much impedance.

My first thought was to stick a couple of low-Ohm power resistors in series with the speaker. (Yes, I know that's adding resistance, not impedance, but my concern is protecting the amplifier, not the frequency response.) The problem there is that they stand almost no chance of finding such a thing in their town (a tiny northern logging town) before Thursday, when the district science fair is.

Next thought: make another coil like the one they have and put it in series. Problem: procuring another magnet in time, plus it might work and might not.

Current thought: put an incandescent light bulb in series with the speaker coil. It should reduce the volume somewhat and mess with the frequency response, but that's no problem. This is proof-of-concept only and really only has to last long enough to be judged. It would be cool if they could let it run the whole time but really it just has to prove to the judges that it will do what it's intended to do, which is act as a transducer.

I told my sister to hit the junk shops and try to pick up another stereo or two for the science fair as backups, but any thoughts on how to protect the amps would be great.

Again, it doesn't have to be loud, and doesn't have to sound good. It just has to make noise while the judges are there, and hopefully not kill the amp it's connected to.

So. Thoughts? I currently think the light bulb is the best bet. I was thinking a regular old truck taillight bulb ought to do the trick, and they could find one easily.


Thanks hugely for any ideas,

Torben

 

[Techie7]

Torben,
It seems to be simple and best would be to make another coil for impedance say between 4ohms to 8ohms. The low impedance might have blowed up the amps. A balloon won't give a good response but a styroform or similar flat tray with a 'z' like flexible cardboard structure attached to two sides will execute a more pleasing sound. 25 turns on a 2.5" diameter is just like short circuit. I think at least 50 turns are needed. I made a balloon speaker when I was child with my own idea. But poor audio response as it is too flexible to pull in/out air. A nice article:
Hi-fidelity Homemade Loudspeaker. From the Inventor of the Styrofoam Plate Speaker, here are the instructions to build the "Hi-Fidelity homemade styrofoam-plate speaker". (8/6/2007)

 

[Torben]

Torben,
It seems to be simple and best would be to make another coil for impedance say between 4ohms to 8ohms. The low impedance might have blowed up the amps. A balloon won't give a good response but a styroform or similar flat tray with a 'z' like flexible cardboard structure attached to two sides will execute a more pleasing sound. 25 turns on a 2.5" diameter is just like short circuit. I think at least 50 turns are needed. I made a balloon speaker when I was child with my own idea. But poor audio response as it is too flexible to pull in/out air. A nice article:
Hi-fidelity Homemade Loudspeaker. From the Inventor of the Styrofoam Plate Speaker, here are the instructions to build the "Hi-Fidelity homemade styrofoam-plate speaker". (8/6/2007)

Thanks for the tips. I was also thinking maybe more turns but I wasn't sure. My only concern is how much speaker cable they have available.


Thanks again,

Torben

[Edit: I just saw the article link you added. Thanks for that! I don't know that it'll help in this case, but I'm glad to have it anyway.]

 

[Mikebits]

We are not talking lots of power right? Why not burn some of it into a series resistor?

Not an elequent solution but for a one time deal...

 

[flat5]

You say speaker wire but we have no idea of the gauge or copper content. If they can get thin flexible wire and use 50 or more turns that may be ok. If they ask around, they may find someone with a multi-tester...like a gas station/service shop. Then they can measure the resistance and get an idea of how much wire to use.

On the other hand, a bulb inside the balloon might add to the project's appeal if it should light at all.
It will tend to average the already low sound level.
Making a resistor of a few ohms might work better.
A very short piece of graphite from a mechanical pencil and two paper clips...or...

 

[stevez]

If my memory is correct the impedance of a speaker is determined at 1 kHz - the DC resistance is likely to be considerably lower. I have measured speaker impedances with my audio generator and measurements compare well with the nominal speaker impedance.

If you had the time, find out what wire they are using and experiment yourself with number of turns, diameter, etc. You might then offer suggestions so that you can avoid the "resistive" approach. Getting the most power to the balloon speaker is probably best.

You might also find that within the limits of their resources that you'll still need to add resistance - the bulbs are good - so is the graphite pencil. Elements from a discarded toaster might also be acceptable. You might measure or estimate the cold resistance of some common automotive lamps as a way of narrowing the possibilities.

Sounds like a fun project.

 

[audioguru]

An incandescent light bulb has a very low resistance when it is cool. Its resistance is fairly high when it lights at about 1000 degrees C.

An 8 ohm speaker has many turns of fine wire so that the coil measures about 7 ohms DC, not a dead short like lamp cord wire.

 

[duffy]

Yeah, that wire is way too heavy. Have him take apart a little transformer or motor or something and use the fine enamel-coated wire inside. Should make for a more powerful balloon.

 

[Torben]

Thanks for all the replies, guys!

Unfortunately I had already had to discard some of the ideas (even though they're good) such as using something closer to proper wire or using a power resistor due either to the fact that this has to be for tomorrow morning and they don't have much spare time, and any solution has to use things around the house. They have taillights, they probably don't have a spare transformer and certainly they sadly don't have time to figure out how to take one apart.

So far I think the best idea (and I mean "best" not in the technical sense but "best" in the "they might stand a chance of managing it by tomorrow" sense) is to double the number of turns and throw a small light bulb in series. I measured a taillight lamp last night at just under 5Ω cold so that might do the trick.

I also really like the graphite idea mostly because it will look totally MacGyver. I'll mention that to them and see whether they can make it work.

Thanks again for all the suggestions!


Regards,

Torben

 

[Video Warrior]

I suspect that the described balloon speaker will indeed continue to be a "stereo killer" without something changed. (added resistor, lamp, lots more turns of smaller wire, but something.)

 

[Torben]

Heh. I know it's been a while but I figured I'd at least close this off with the final results.

My sister managed to pick up 4 1Ω resistors at the local shop and wire them in series with the coil, and they doubled the number of turns as well. However, they couldn't find another stereo donation in time for the science fair, so the thing never actually operated again. The judges did know that it had in fact worked in the past though, and understood why there was no live demo, and my nephew managed to finish 3rd overall despite not having a working project. Not bad at all.

Thanks again to all who replied! I (we) appreciate it.


Cheers,

Torben