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Old 3rd March 2004, 08:11 AM   #1
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Default Aluminum foil used as a transducer?

Could aluminum foil (like cooking foil) be used as a transducer? I mean since most ribbons and magneplanars use aluminum strips attached to a mylar or similar membrane. So could use just forgo the mylar and use the foil since the entire surface is aluminum. Although you wouldn't be able to stretch it like mylar thats for sure.


Anyways any ideas on this. Oh ya one question, a fundamental one I suppose. Since aluminum isn't magnetic, how do ribbons and stuff work exactly? Does the electromagnet affect aluminum differently than a earth magnet?
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Old 3rd March 2004, 08:27 AM   #2
sreten is online now sreten  United Kingdom
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The simple answer is yes it can. It is available in various
thicknesses, obviously usually the thinner the better.

Aluminium is a non-magnetic conductor similar to copper.

Copper is a a good conductor, but conductance/weight aluminium
wins out. So Aluminium is used in speaker voice coils when light
weight is required and is the general choice of conductor for
panel and ribbon drivers.

Even with a stepdown transformer working at low resistances
can be a nightmare due to contact resistances . So commonly
the foil is slitted and mylar or similar backing added.
Either to create a higher resistance diaphragm and / or to
control the current directions in a magneplanar type driver.

sreten.
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Old 3rd March 2004, 08:51 AM   #3
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What's less known is that aluminium has much worse mechanical yield and fatigue properties than copper. As well as that, to get the same conductivity with aluminium as with copper, the aluminium foil has to be much thicker and that compounds the problem.

Whatever the material, the conductivity has to be good because the amount of current flowing through the magnetic field is a major factor that determines how loud the speaker's gonna be. And the magnetic gap is extremely wide and often requires massive neodymium magnets. Because of the super-low resistance of those solid metal ribbons they generally have big transformers hanging off the back so you can operate them with ordinary amplifiers.

CM
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Old 3rd March 2004, 09:34 PM   #4
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What exactly makes the foil diaphragm blow up when you feed too much current through it? I know it's the same thing that happens to single-use fuses; is it the heat? Or is it something else?
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Old 3rd March 2004, 11:16 PM   #5
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Default ADHEARING ALUMINUM FOIL TO POLYIMIDE OR KALADEX

Anyone know of a low weight technique for strongly adhearing aluminum foil to a plastic film like polyimide or Kaladex?

Normal adhesives appear to require 25 to 50 microns of thickness. Most ribbon aluminum foils are 5-9 microns thick.
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Old 3rd March 2004, 11:25 PM   #6
Magura is offline Magura  Denmark
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You can glue PA with triclorethane, it dissolves a thin layer of the PA and adheres it to the aluminium.


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Old 4th March 2004, 02:01 AM   #7
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Yes, aluminum foil can be used as a membrane. Your problem will be getting enough resistance. Ordinary kitchen foil is too thick and too wide. Narrower and thinner is the ticket. You'll need to think in terms of a transformer if you can't get the resistance up high enough.
You'll need strong magnets and they'll need to be close together so that the flux lines are reasonably straight, otherwise you'll get high distortion from the driver itself. It also increases the field strength, making the driver more efficient. Having the magnets close together goes hand in hand with having a narrow tweeter element, but doesn't work too well for midranges and very poorly for woofers. This is why you keep hearing about ribbon tweeters, but not ribbon subwoofers.
In the Magneplanar ribbon tweeter there is no Mylar or other plastic, just the aluminum. The aluminum is run through a machine that crimps it so that the actual element has tiny ridges in it; they run across the element, not down the length.
Other drivers do use various plastic backings.
True, aluminum isn't magnetic, but it doesn't need to be for the same reason that copper voice coil wires don't need to be. The driving force comes from the magnetic field created by the audio signal. This interacts with the fixed magnets' field to move the ribbon. In fact, using a ferro-magnetic wire (or ribbon elements) would create distortion, since the wire would magnetize and hold the field through hysteresis, giving the signal a hangover effect.
For what it's worth, I have made drivers out of the aluminized plastic from an old Wondercap. Yes it worked. It wasn't very efficient, but then I was using a relatively wide element (about an inch, as I recall). The aluminum coating is very thin indeed and has a fairly high resistance.
Oddly enough, one of your biggest problems will be getting the signal into the aluminum. The easiest method is to clamp something to the aluminum. It's not as though you can solder it and by the time it gets thin enough to do you any good, it's really, really delicate.

Grey
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Old 4th March 2004, 03:12 AM   #8
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You may want to experiment with using a transformer from a soldering iron (one of the ones that have a U-shaped wire for the tip), but I don't know if the result would be any good as it's designed for 50-60Hz. You could try etching away at the foil with caustic soda or something like that to make it thinner. Alternatively, look for some techniques used for DIY electrostatics: such as graphite powder pressed into soft plastic foil or similar, or electroplating. How about a conductive spray or antistatic spray?

If you have a look at some commercial ribbon tweeters, you'll see that most of them have bulky transformers that are huge compared to the neodymium magnets that they use. If you go in the other direction for extreeeeeemmmeeely thin foil, you might have consistency problems and might never get a matched stereo pair.

CM
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Old 4th March 2004, 04:51 PM   #9
medum is offline medum  Denmark
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Default Re: Aluminum foil used as a transducer?

Quote:
Originally posted by Hybrid fourdoor
Could aluminum foil (like cooking foil) be used as a transducer? ......

Yes, but it's right - resistance would'nt get low enough, unless you split up the surface to a number of long small strips and connect in serial configuration - like in the tweeter you see on my apogee-clone:System Pictures & Description
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Old 4th March 2004, 04:58 PM   #10
SY is offline SY  United States
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Quote:
How about a conductive spray or antistatic spray?
Way too high ohms per square.

Quote:
Anyone know of a low weight technique for strongly adhearing aluminum foil to a plastic film like polyimide or Kaladex?
You can get Kapton tape with very thin adhesives. A heat-seal type would be the lowest weight. Now, getting full sheets might be a challenge if you're not buying quantity, but all you really need to do is back up the slits in the aluminum foil.
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