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Old 13th June 2008, 11:43 AM   #1
APi is offline APi  Finland
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Default Tubular magnetic

I have been thinking lately a lot about a concept where ribbon would not be flat but a tube instead. Hanging between magnets as free as possible. Only attached to elastic wires on top.

Itīs not planar magnetic but more like tubular magnetic


+ Tube is more rigid than flat ribbon.
+ Curved surface wonīt get noisy wrinkles as easily.
+ Air gaps can be smaller since there is a place for a steel rod inside the tube!
+ Therefore a tube can be wider.
+ Itīs polar response is still pretty flat.

- Tube resonances are a bit problematic. However a free hanging tube is not forced to twist as much as Linaeum tweeter, some stuffing can be placed inside the tube (not touching the tube) and surface joints can be cut to form a kind of diffusor for surface acoustic waves. These can help a bit if internal damping of used material is not enough.

I think a thin paper & coil wires would be a good stating point. Works somehow...
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Old 17th June 2008, 12:07 PM   #2
owen is offline owen  United Kingdom
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Ok lets take this from the top.

It will have a high mass.
So efficiency will be low.

Tubes have bending moments - and as they are stiffer, and the mass is higher, as a consequence the resonance will be bigger and uglier.

Having said all that, I have contemplated a multiple parallel magnet wire stator, but the magnetic circuit gets tricky. Especially to avoid the obligatory line array combing that would arise from such a package.

An interesting thought, but do try it and see.


Owen
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Old 17th June 2008, 01:54 PM   #3
APi is offline APi  Finland
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Quote:
It will have a high mass.
So efficiency will be low.
= 3.14x more mass compared to a ribbon of the same thickness.

Donīt make sense if the tube has small diameter. But if midranges need to be produced and therefore tube must be wider (>1") then perhaps... The wider tube is, the less motion is needed and therefore air gap between magnets and internal steel core can also be made thinner. Steel core can be something like wire mesh combined with stuffing material needed for damping high frequencies.

Quote:
Tubes have bending moments - and as they are stiffer, and the mass is higher, as a consequence the resonance will be bigger and uglier.
Possible yes, but a tube can also be made of thinner material than a planar: Tube is rigid and immune against wrinkles.


Quote:
magnetic circuit gets tricky. Especially to avoid the obligatory line array combing that would arise from such a package.
I am actually looking for a line array

One other interesting thing is that if the whole surface of the tube carry current then we have basically "infinitely long voice coil": Almost all magnetic flux goes through the wire and motion is very linear. A kind of push-pull structure deleting even order harmonics. And slightly varying field vector can actually compensate bending moments a bit. Electromagnetic force spread the tube where mass and momentum squeeze it. Partially.

Eddy currents and sensitivity are another story...
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Old 18th June 2008, 10:54 AM   #4
owen is offline owen  United Kingdom
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Quote:
a tube can also be made of thinner material than a planar:
A 5 micron tube. Good luck finding one, and better luck getting it shipped.

Quote:
Tube is rigid and immune against wrinkles.
Not quite - It is slightly more rigid than the equivalent rod of the same section - this pushes the resonance up and also the Q.

The tube would be a pure line array in one dimension, but due to efficiency loss, it would need to be paralleled - and that is a more normal 'multidriver' line complete with a full set of undesirable side effects - combing, restricted off axis response etc. etc.

However, this could be a good way to get a bass/midbass package with similar sonic character to a 'normal' ribbon - the ferquency of operation making design and build more straightforward, and avoiding the 'nasty' side effects of a high power low frequency ribbon.

A mockup could be made using plumbing copper pipe, some reasonable magnets, and some steel bar.

The center 'flux bridge' does not need to be a rod - a flat bar will do nicely.

I'll see what I can mock up, and blast it to get an FR (and a measure of relative efficiency)... but that may take a while, as I am time poor...


Do build your own prototype, and report back.

Owen
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Old 18th June 2008, 02:08 PM   #5
APi is offline APi  Finland
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Quote:
A 5 micron tube. Good luck finding one, and better luck getting it shipped.
There is no such thing available... Isnīt there?

As long as itīs tubular magnetic (wires glued on a foil) then glue can also be used to make a tube out of foil. No big sonic issue. Itīs already not perfect.


Quote:
However, this could be a good way to get a bass/midbass package with similar sonic character to a 'normal' ribbon - the ferquency of operation making design and build more straightforward, and avoiding the 'nasty' side effects of a high power low frequency ribbon.
Exactly. Getting good high frequency response is tuning, failing, tuning, failing, tuning, failing, ... But we donīt know if we donīt try.

Quote:
A mockup could be made using plumbing copper pipe, some reasonable magnets, and some steel bar.
Copper pipe... Uh... Too heavy...

Quote:
The center 'flux bridge' does not need to be a rod - a flat bar will do nicely.
Or acoustically open mesh with some stuffing? Somehow I donīt believe that hollow tube without any damping material is good enough.

Quote:
I'll see what I can mock up, and blast it to get an FR (and a measure of relative efficiency)... but that may take a while, as I am time poor...

Do build your own prototype, and report back.
Thanks. Thatīs what I liked to have with this topic. I also have lack of time (weddings coming etc) so any new findings from as many people as possible is great.
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Old 19th June 2008, 03:33 PM   #6
owen is offline owen  United Kingdom
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Hi,

Yes the copper would be heavy, but it will move the overal system resonance lower. Especially as the system compliance will be high.

And most importantly, its what I have around at the moment.

Thought of the moment - gently flatten the tube - it'll work harden the sharper corners on the elipse that is produced, making it stiffer still.!

I'll see what I can bodge together as a prototype.

Have fun


Owen
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Old 25th August 2008, 03:48 AM   #7
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Join Date: Aug 2008
Default tubular tweeter

Long ago I began assembling this tweeter, but never got past the one third stage.
I took plumbing pvc pipe drilled it with 3/8 inch holes. Stuff it with cotton. Glue a wire winding around the pipe but trying not to cover any holes. fix your spacers to the pipe. My thought was to use several runs of vcr tape for the charged membrane. Now you have to find a slightly bigger pvc pipe (with holes drilled in it) and a wire glued in the inside.
Any ideas on wheather this would work?
Paul
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