Importance of magnetic field homogeneity for ribbons? - diyAudio
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Old 20th July 2011, 06:00 PM   #1
brunob is offline brunob  Germany
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Default Importance of magnetic field homogeneity for ribbons?

It is often reported that the magnetic field in the gap should be as homogeneous as possible. In this context homogeneity means that the strength of the magnetic field in the middle of the ribbon is the same as the strength of the magnetic field in the edge of the ribbon.

To quote the RAAL litterature:
"The magnetic field in RAAL ribbons is homogenous. Regarding the width of the gap, it is designed to have the same strength at the ribbon edges, as well as at the ribbon middle portion. It reduces the stress at the ribbon foil dramatically. The distortion, too. To get the precisely shaped field, through the gap width and depth, we’ve extensively used the FEM and BEM magnetic circuits analysis software. In order to obtain such a field in real magnetic circuit, very strong NdFeB magnets have been precisely machined, prior to magnetization, in very special shape. Making the magnetic field homogenous, sacrifices the strength of it, but efficiency should easily be changed for low stress and low linear and nonlinear distortion. " RAAL Ribbon Tweeters


The "The tall neodymium ribbon". Part 1 uses a design with a deep cavity and multiple magnets in order to obtain an homogeneous magnetic field.


On the other hand, others use wide ribbon with narrow magnets which can weakened the middle. For example 0.6my aluminium ribbon.

In all cases, very good sound is reported.

I have built a 20 cm long tweeter ribbon with a 2.2 cm gap that had only 50% strength in the middle of the gap. I was surprised how good it sounded.


I wonder, if an homogeneous magnetic field is important to get a good sounding ribbon?


Bruno
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Old 23rd July 2011, 06:06 AM   #2
a.wayne is offline a.wayne  United States
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Not necessary for good sound , but i can see where it would make for " better sound " ..

I'm currently looking into this myself as my mid/ribbon is 46mm wide and would love to increase the force in the middle for lower distortion and increased efficiency . The cross bar concept was mentioned by another here , but I'm not in full understanding of how to implement such a setup or if it really works as stated ..

Interested if you continue with this ....................
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Old 23rd July 2011, 06:27 AM   #3
a.wayne is offline a.wayne  United States
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Here is Peter's explanation for helper magnets ...

Quote:
Originally Posted by UKPD View Post
Hi A.Wayne
helper magnets idea came from guys at e-magnets.co.uk, they suggested to close the magnetic path between two steel bars (distance setting, so eg. 40mm wide ribbon between 10mm thick neos will require 60mm thick stack of helper neo's) with a stack of magnets as they will help to alleviate field strength drop in the middle of the ribbon length. In effect additional field off helper magnets will add to the field, observe the polarity thou, it has to be reversed in relation to front magnets! I will post a picture tomorrow.

Peter
Not sure if you can simulate in Femm .................
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Old 23rd July 2011, 11:28 AM   #4
UKPD is offline UKPD  United Kingdom
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Hi guys,

I'm not sure if you can simulate it, but in reality adding helper magnets did increase sensitivity on the ribbon I added them on. No measurement unfortunately. I'ts easy enough to try though. Another interesting point to consider would be to replace all of the distance blocks with stacks of magnets. that would give you higher sensitivity,and no field decrease in the middle. If you have Gauss meter that would be interesting to measure the effect of adding the magnets. The way I see it is like adding strength to the main magnets.

Peter

Last edited by UKPD; 23rd July 2011 at 11:29 AM. Reason: correction
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Old 23rd July 2011, 02:31 PM   #5
a.wayne is offline a.wayne  United States
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Peter according to your diagram you are using them in the middle only .. correct ..?
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Old 23rd July 2011, 08:24 PM   #6
UKPD is offline UKPD  United Kingdom
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Sorry about the delay,work getting in the way of hobby(nuissance)

Yes ATM I'm only using a stack in the middle, I've got some more magnets coming next week, so will try adding them at both ends, see if it makes a difference worth extra cost.

Cheers

Peter.
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Old 24th July 2011, 04:06 PM   #7
brunob is offline brunob  Germany
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Quote:
Originally Posted by UKPD View Post
Sorry about the delay,work getting in the way of hobby(nuissance)

Yes ATM I'm only using a stack in the middle, I've got some more magnets coming next week, so will try adding them at both ends, see if it makes a difference worth extra cost.

Cheers

Peter.
A picture would be welcome.

I have a gaussmeter and if I can reproduce your setup I would measure it.
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Old 24th July 2011, 07:49 PM   #8
UKPD is offline UKPD  United Kingdom
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Helper magnets picture. Not elegant solution, just a quick test. Let us know if you see any measurable difference.

Peter.
Attached Images
File Type: jpg ribbon magnets 1.JPG (93.7 KB, 156 views)
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Old 24th July 2011, 08:56 PM   #9
a.wayne is offline a.wayne  United States
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lucky you tight fit .......
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Old 24th July 2011, 09:08 PM   #10
UKPD is offline UKPD  United Kingdom
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Regarding field strength drop in the middle, in my setup which is 1200 long ribbon (on 1300mm long steel bars 20x50mm, 40mm gap with N42 grade magnets 10x20x40mm) I get 0.15 T in the middle and 0.177 T ends. I have two steel bars at both ends to close magnetic circuit, same size as main bars, 20x50mm mild steel. Unfortunately I had to return the meter before I had a chance to measure the effect with the helper stack.

Regards, Peter.
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File Type: jpg ribbon end.JPG (376.8 KB, 144 views)
File Type: jpg ribbon center.JPG (394.0 KB, 138 views)
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