field strength of magnets vs. electrostatic - diyAudio
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Old 2nd January 2015, 02:11 AM   #1
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Default field strength of magnets vs. electrostatic

Hi,
I can't find a direct comparison of the strength of magnets in, for example a ribbon tweeter (~.6 -.8 T usual ((?)) ) to a a.c. signal in the ribbon of only a few volts vs. the field strength exerted on a charged membrane of approx. 4000 V in an electrostat say 2 mm. away from wires spaced at 1/8 inch carrying a a.c. voltage increased by a transformer (X10 ((?)) ).
When considering the input voltage from the transformer (input voltage?) is increased manyfold, and the field charge of the membrane is almost at the ioniaztion of air, :
WHY ARE RIBBON TWEETERS SO MUCH HIGHER IN EFFICIENCY THAN ELECTROSTATICS?

Or, put another way, what is the field strength in Tesla of a (for example) acoustat loudspeaker?

Thanks,
Paul
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Old 2nd January 2015, 07:04 AM   #2
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This is something that I have always wanted to figure out too.
I have found all of the math but never took the time to do the math.

Basically a place to start is to choose what your magnetic field strength will be, as .6t to .8t is fair amount of difference of 1.33 more force.
This is a bit less than the square root of 2 (1.414) or roughly half of double, so this will make a difference of somewhere in between 2db to 4db (2.5db?) in the efficiency just in the two levels of Magnetic Field strength.

I apologize for not being exact and not taking the time to properly work this out.

http://www.sengpielaudio.com/calculator-db.htm


Then you must convert the voltage applied to the ribbon into current, as its magnetic field is directly related to the amount of current flowing in the element.

I have seen this part of the formula in a couple of these threads.

The results well come out as force acted upon the ribbon and typically will be in Newtons or Dynes.

http://www.unitconversion.org//unit_.../force-ex.html

Then Dynes gets converted to SPL effeciency in db for a given power level.

Then you can do the same for Electrostat's using the proper equations to compare as well.

I think that I had read in Peter walkers papers that this comes out to be around a peak of 50 Dynes per surface area (1 square meter, I think) or so so and the peak force is limited by the ionization, or breakdown voltage, of the surrounding air.

https://books.google.com/books?id=lP...ations&f=false

here is a nicer link,

https://docs.google.com/file/d/0B_l7...zY2/edit?pli=1

That is as far as I got, but I hope it helps you to understand a little more.

jer

Last edited by geraldfryjr; 2nd January 2015 at 07:32 AM.
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Old 2nd January 2015, 08:11 AM   #3
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In the book it was stated as 50N/m^2 and not 50 Dyne's as I had stated, sorry for the mis-information.

Here excerpts of the pages of that particular section on just the ESL transducer and its forces.
The Magnetic stuff is in the Book as well in another chapter and an even more in depth description in the section before this one..

What an Awesome book it is.

I found those links freely doing a Google search so I hope that there won't be any copyright issues with my post.

jer
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Old 7th January 2015, 03:22 PM   #4
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Default Thanks Jer

I guess I'm always trying to reinvent the wheel here, but the gist of my question is about weather you could make a ribbon type speaker with charged plates instead of magnets. So, you would have a high voltage applied to front stators say +5000V and a high voltage applied to the rear stator say -5000V
and a ribbon tweeter hanging in between.
Is this more powerfull, than neodymium magnets to the right and left of a ribbon tweeter.
This was the extent that I would take it,as I was thinking of Acoustat type panels to the sides of these for the midrange and bass.
However, if it is possible to work this arrangement, I would think the best application of this would be on a full range ribbon (a'la Apogee)?
Or maybe I'm way off base here?
Thanks again
Paul
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Old 7th January 2015, 11:06 PM   #5
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There are some accordion shapes used for planar drivers but the accordion principle is introduced to solve other limitations, not typically because it is a good idea in and of itself.

If you have electrostatic fields, easy enough to shape them over large areas and use flat tight diaphragms to make the sound vibrations without inroducing the complexity and irregularity of accordion shapes.

Ben
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Old 7th January 2015, 11:07 PM   #6
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Some of us have been asking about field magnets for cone speakers using electro-magnets, as in days of yore. With contemporary rectifiers and capacitors, a piece of cake to construct now.

When the question was aired a few months ago at DIYaudio, I seem to recall nobody thought you could best a permanent magnet for a cone kind of driver.

Could electro-magnets be feasible for planar drivers????

Ben
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Old 7th January 2015, 11:22 PM   #7
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In my opinion, no. NdFeBr magnets have flux density @ 15kGauss and electromagnets can not exeed that by much at any current with a soft iron core. Neo magnets are the way to go here. Electrostatic force potential is weak in comparison and highly nonlinear over large excursions.

Though maybe if you were really crafty you could employ both EM/ES drive.

Last edited by Andrew Eckhardt; 7th January 2015 at 11:26 PM.
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Old 8th January 2015, 05:30 AM   #8
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I have thought about making a true ribbon ESL, But I don't see the benefit with it.

Complexities can arise do to lack of support of the diaphragm and this will lead into stability issues of it sucking in to one of the stators.

However I think with enough tension it can be done.
I have burned plenty of, and, big enough holes in some of my diaphragm to warrant them practically operating in a true ribbon fashion!!

After a while at the edges of the holes would cause tears, and then, they would start to rattle badly and get worse to the point that I would finally have to replace it.

Also in order to possibly make say a 2 or 3 inch wide true ribbon one would have to use a thicker material in order to maintain enough even tension at only to points (at the ends) and integrity of the diaphragm flatness.

You can forget about using pleats as well, as the surface irregularity's and lack of mechanical stability will cause it to suck into one of the stator's.

IMHO, Because of all this, Using a heavier material you are taking away from the one major benefit of an ESL driver, and that is being able to have the lightest diaphragm in the world only second to a Plasma Driver!!

However I think it would be a interesting experiment just to see how well it would work.

I had actually started a similar design with a 1/2" to 1" ribbon, I routed out 12" long frame out of wood and heavily coated with some Poly but I never finished it.

It wasn't until 2003 when I built my very First panels that I found out exactly what kind of voltage we are dealing with and how to contain them!!!

Even then I had planned on 10Kv to 20kv in my designs and builds but only one set actually made to that range, as most of you already know.

Here is another Genie in a Bottle idea that I have been thinking about, is to use a rather thick piece of Mylar or Acetate or some kind of plastic sheet of say .005" to .020" and suspend it in a frame true ribbon style using a rubber band suspension like the way the Epsilon's are constructed.

This may work good for low to mid frequency's as it would simplify the need of having high strength rigid frame due to the tension needed to make a rigid and uniformly flat diaphragm with thin material.

Put it in a strong HV field and a rather large D/S of say .125" or even a dual diaphragm compounded system.

If such a system could work it would take very little power to drive, maybe even possibly a method from a form Direct Drive !!!

I am for any ideas to get some kind of bass out of them and eliminate the costly and bulky transformer.

jer
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Old 8th January 2015, 07:20 AM   #9
jcx is offline jcx  United States
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actually the problem is that electrostatic forces are so huge - high enough electric fields to start giving big force on your membrane quickly overcome intervening dielectric's strength, they tear apart the molecules in between in sparks, discharges that then provide a conductive path and let the separated charges get back together
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