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Old 17th July 2006, 12:50 PM   #1
APi is offline APi  Finland
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Default Induction planars

I just read about induction AC motors. The same principle could be used with planars?

My idea is to combine a membrane made by thin aluminum foil and electromagnet generating variable magnetic field through the foil. Variable magnetic field generates eddy currents on the foil and move it. The foil can be mounted several ways to minimize resonances.

No need for expensive magnets. Possible with good sensitivity, I doubt. But anyone tested this?
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Old 17th July 2006, 02:36 PM   #2
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Hi Api,

i doubt that induction AC-motors will be appropriate for a sound-transducer.

On one side they have strong coils to provide high force, but the coils are really heavy weight, compared to a voice coil of a dynamik speaker.

I have some experience with those engines, since i developed NC-machines using this technology. The performance is incredible and for frequencies lower than about 60 Hz it would be possible to drive ANY membrane, noway which weight it got. Thos engines can accelerate up to 50 times the gravity !!

capaciti
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Old 17th July 2006, 06:05 PM   #3
dnsey is offline dnsey  United Kingdom
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Quote:
i doubt that induction AC-motors will be appropriate for a sound-transducer.
I don't think he means to use an actual motor (as in rotary machine), but to design a speaker using the same theoretical principles.
Sounds like an interesting idea!
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Old 17th July 2006, 07:48 PM   #4
APi is offline APi  Finland
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Quote:
Originally posted by dnsey

I don't think he means to use an actual motor (as in rotary machine), but to design a speaker using the same theoretical principles.
Yep.

I made a quick test. Used a few inches diameter coil having 10 turns. It was glued on paper. Then put small piece of aluminum foil on it. Drove the coil with my audio amplifier.

I got small sound out of the coil on paper. However adding aluminum foil didnīt increase volume at all. Maeby you really need to have huge currents or much more turns.


But anyway... this was just the first test.
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Old 17th July 2006, 09:19 PM   #5
dnsey is offline dnsey  United Kingdom
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You need to apply the 'left hand rule' (I think: I'm rather shocked that I don't remember such a fundemental thing clearly ).
Anyway, basically the foil needs to be at right angles to the coil diameter in order for the fields to interact.

OTOH, perhaps I'm envisioning your setup incorrectly!
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Old 18th July 2006, 08:04 AM   #6
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A induction treble unit was described 1961 in an American DIY
magazine, dont remember which, and the same speaker was
later patented.

I shall see if I can find the patent number.

It has an interesting property, the eddy currents that is induced in the aluminium sheet/membrane forces it to exactly follow the
signal input!
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Old 20th July 2006, 04:03 PM   #7
el`Ol is offline el`Ol  Germany
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Inductive drive units were very common in Germany in the 1920ies and 1930ies. These systems (called Freischwinger) were used for the mass market, in contrast to the sonically superior dynamic drives. There were even planars, similar to the NXT speakers in our times.
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Old 25th July 2006, 10:17 AM   #8
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The patentnumber of the Induction Planar is US3198890 and
the magazine with the article about the unit was
"Electronics" of June 16, 1961.

You can fetch the patent here: http://free.patentfetcher.com/
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Old 25th July 2006, 11:08 AM   #9
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don't know about planars but this speaker's supertweeter is an induction dome:

http://www.stereophile.com/floorloudspeakers/391/

Regards

Charles
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Old 16th August 2006, 12:30 AM   #10
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Default Double the Hifi?

A true magnetless induction transducer is unfortunately a frequency doubler since it has no way of discriminating the polarity of the induced field (which is why a permanent magnet is needed as a reference unchanging field).
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