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Design Consult on Headphones

post #1 of 24
7/5/11 at 1:29pm
Thread Starter 

If you'll excuse the lengthly post, I need some assistance finishing the design on a pair of headphones so I can buy parts, if I will be able to afford them. 

 

The principle is to effectively create a pair of electrostats that use magnetic rings rather than stators, and have a floating diaphragm (suspended in the air by magnetism) that is pushed back and forth by changes in the electromagnetic field on either side of the diaphragm. There are four ring magnets, two in the center attracting each other, and two on the outside repelling the ones in the center. The thing is, if one magnet ever attracts the center pair of magnets, they will "stick" to the magnet that is attracting them until the voltage flips and they are attracted to the other side. This is a no no unless triangle waves  are a "good thing" to produce when the headphones should be making sine waves. The solution That I've come up with for this issue involves putting an electromagnet in front of the outer permanent magnets, with them facing opposite directions. When the voltage is positive, the positive electromagnet and the positive permanent magnet work in tandem to create a strong repulsive magnetic field on one side, and on the other side, the attracting electromagnetic field and repelling permanent magnet field should produce a less repelling (so long as its not attracting) magnet field. Confirm / Deny? If this works, than I just need some pointers on stretching 1 mil mylar out with 150 pounds of tensile strength.

 

Thanks

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post #2 of 24
7/5/11 at 5:15pm

 

You're not going to be able to suspend the diaphragm in the air by magnetism.

 

se

 

 

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post #3 of 24
7/5/11 at 6:21pm
Thread Starter 

Hm, have a diagram - 

 

+          -

1   + -   2

     + - 

+          -

1          2

 

The pluses with 1s under then represent the inwards facing side of the first ring magnet, particularly, the upper and lower limits of the ring when viewed from a profile direction. Same for the minuses, just with a different polarity. The center four symbols show the poles of the smaller inner ring magnet. shouldnt the diaphragm in the center be suspended by the lower "+1" and the lower "-2"? the ring is positioned in a direction facing upwards from these rings, and while the same force will be applied to the other side, gravity will only bring the ring down so far before the magnetic repulsion of the lower section of the rings is equal to the diminish magnetic repulsion of the upper section of the rings, plus gravity. We're talking strong magnets here, N40 neodymium.

 

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post #4 of 24
7/5/11 at 9:31pm

 

Won't work. No simple, static array of magnets are stable. Doesn't matter how powerful they are. See Earnshaw's theorem.

 

se

 

 

 

 

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post #5 of 24
7/6/11 at 7:52am
Thread Starter 

What about those japanese trains? Don't they levitate with magnetism?

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post #6 of 24
7/6/11 at 9:22am

 

The electromagnetic systems use corrective feedback to maintain stability. And the electrodynamic systems aren't static.

 

se

 

 

 

 

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post #7 of 24
7/6/11 at 12:19pm
Thread Starter 

Well in layman's terms, what makes it impossible for a static dmagnetic system to create levitation?

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post #8 of 24
7/6/11 at 12:31pm

http://en.wikipedia.org/wiki/Earnshaw%27s_theorem

 

it is basically impossible to describe this phenomenon in "layman's terms"...  i suppose you could think about it as stationary ferromagnets not being able to constantly hold a perfectly stable field - there will always be an instability in any one direction

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post #9 of 24
7/6/11 at 1:43pm

I could be reading this wrong, but if the magnets are meant to attach to the diaphragm the moving mass will be extremely high and you will find exceptional difficulties getting good high-frequency response. The point of planar drivers is that they have very low moving mass; attaching even a small magnet to the diaphragm totally defeats that.

 

Tangentally, I dont think Earnshaw's therum applies here. The diaphragm will maintain equilibrium between the magnets. I do however agree that not much will happen after that.


Edited by nikongod - 7/6/11 at 1:46pm
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post #10 of 24
7/6/11 at 3:01pm
Thread Starter 

The mass of the magnets should be about 20 grams for the magnets I would be attaching. The material I'm using is .001 inch thick mylar in the hopes of countering this, still way too high?

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post #11 of 24
7/6/11 at 3:48pm

Just a SWAG here, so don't let me poop on your magnetic party, but I would think that anything more than a few milligrams would have an inertia so difficult to control that any benefits would be eaten up by the power required to control it.

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post #12 of 24
7/6/11 at 4:37pm

20 grams is a lot of mass to move back and forth at 20 kHz.

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post #13 of 24
7/6/11 at 6:07pm

 

Quote:
Originally Posted by digger945 View Post

Just a SWAG here, so don't let me poop on your magnetic party, but I would think that anything more than a few milligrams would have an inertia so difficult to control that any benefits would be eaten up by the power required to control it.

 

Quote:
Originally Posted by bcg27 View Post

20 grams is a lot of mass to move back and forth at 20 kHz.

 

I agree with both. 

 

I guess Im kind of lost here because I dont actually know how much moving mass there is in a conventional dynamic driver. Instinct tells me 20g is very heavy though. 

 

I have a busted  HD650driver I will happily sacrifice to science... back in a few.

 

Unbeknownst to all of you since I never posted the first messsage I have been in my secret lair haxoring headphones and all that stuff. 

 

Behold! The driver of an HD650. It is actually quite possible that this driver came from an HD580 or 600...

 

DSC_1828.JPG

 

Snuggled in its shell all nice and pretty. I got this driver and its mate when their former owner broke the acktardedly fine wires leading up to the voice coils. 

 

I carefully cut the driver around its edge. I tried to get as much of the driver as possible, and left VERY little (a few patches less than 1mm wide*4mm long) in the shell. I'd argue ferociously that the outter edge of the driver is not actually MOVING mass because it is anchored to the edge and does not move very much... 

 

Then I plopped it on my digital tracking force scale. 

 

DSC_1830.JPG

 

0.10g 

The weight fluctuated between 0.08 and 0.12g, but its my scale so Im going to go with 0.10g 0.10g is a nice round number so the math will be easy. 

 

20g/0.10g=200 times more weight. Not accounting for any tension in the membrane which the magnets are mounted on. It only takes a few mW to get the HD650 singing proud, Id wager you will need a LOT.

 

Some headphone prons. 

 

The inner-bits of the HD650 magnet:

 

DSC_1834.JPG

 

The voice coil rides in the little slot in the magnet. 

 

A pic of the insides of the driver and the voice coil:

 

DSC_1840.JPG

 

I think Im going to set up my photo-tent and re-shoot this one to enter in a contest. It looks pretty cool IMO. Opinions?


Edited by nikongod - 7/6/11 at 6:52pm
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post #14 of 24
7/6/11 at 6:56pm
Quote:
Originally Posted by nikongod View Post
 
Tangentally, I dont think Earnshaw's therum applies here. The diaphragm will maintain equilibrium between the magnets. I do however agree that not much will happen after that.

 

Can you explain how that could possibly be the case without the diaphragm being connected to something to keep it stable?

 

se

 


 

 

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post #15 of 24
7/6/11 at 6:58pm
Quote:
Originally Posted by nikongod View Post

 

I think Im going to set up my photo-tent and re-shoot this one to enter in a contest. It looks pretty cool IMO. Opinions?


Definitely do!

 

se

 

 

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