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doped cloth surround?
doped cloth surround?
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Old 17th June 2010, 02:06 AM   #1
c3conv is offline c3conv  Philippines
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Default doped cloth surround?

Does any body know what is the meaning of a doped cloth surround? What is used for doping the cloth and what type of cloth are usually used? thanks in advance.
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Old 20th June 2010, 11:02 AM   #2
LineArray is offline LineArray  Germany
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From your post i cannot see clearly what your intention is.

If you talk about the surround of conventional speakers a
treatment of the surround or parts of the cone may be used to

- make the surround airtight, if the cloth used is leaky
- introduce damping to the surround (and maybe parts of the cone)

I do not think there is a general recipe used, every manufacturer
will use its own.

Of course most treatments will usually also introduce stiffness and the
resonance frequency of the system can be expected to increase, since
the surround contributes to the stiffness of the suspension of the cone.

There may be several kinds of resins used, i think in earlier times when
the surround was often made of paper and was one piece with the cone,
also bitumen based paints were used. That coating may also prevent
the surround from ageing if the coating/doping has the right consistency
not to brittle during the time the system is used.


I do not know what is commonly used but

- Poly Acryl
- Poly Urethane
- Silicone
- Bitumen
- Poly Vinyl based

are some stuffs which come to my mind ...

What is important is good robustness against the UV radiation
which comes with the sunlight.

If the meshes in the cloth are very fine, even some kind of oil might be
sufficient to make the surround tight. I think it depends on what you want
to achieve.
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Last edited by LineArray; 20th June 2010 at 11:28 AM.
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Old 20th June 2010, 11:34 AM   #3
LineArray is offline LineArray  Germany
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- Epoxy based
- Rubber Solution ?
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Last edited by LineArray; 20th June 2010 at 11:41 AM.
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Old 20th June 2010, 12:21 PM   #4
c3conv is offline c3conv  Philippines
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Quote:
Originally Posted by LineArray View Post
From your post i cannot see clearly what your intention is.

If you talk about the surround of conventional speakers a
treatment of the surround or parts of the cone may be used to

- make the surround airtight, if the cloth used is leaky
- introduce damping to the surround (and maybe parts of the cone)

I do not think there is a general recipe used, every manufacturer
will use its own.

Of course most treatments will usually also introduce stiffness and the
resonance frequency of the system can be expected to increase, since
the surround contributes to the stiffness of the suspension of the cone.

There may be several kinds of resins used, i think in earlier times when
the surround was often made of paper and was one piece with the cone,
also bitumen based paints were used. That coating may also prevent
the surround from ageing if the coating/doping has the right consistency
not to brittle during the time the system is used.


I do not know what is commonly used but

- Poly Acryl
- Poly Urethane
- Silicone
- Bitumen
- Poly Vinyl based

are some stuffs which come to my mind ...

What is important is good robustness against the UV radiation
which comes with the sunlight.

If the meshes in the cloth are very fine, even some kind of oil might be
sufficient to make the surround tight. I think it depends on what you want
to achieve.
Thanks for your reply. There are two vintage Fostex coax speakers that I can't find the replacement so rather than throw it away I was wondering if I can make my own surround. Since Fostex are quite sensitive I guess putting some fine clothe will be good but what kind of oil would greatly help if you can suggest one.
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Old 20th June 2010, 12:22 PM   #5
c3conv is offline c3conv  Philippines
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Quote:
Originally Posted by LineArray View Post
- Epoxy based
- Rubber Solution ?
What are some example of epoxy based rubber solution?
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Old 20th June 2010, 12:57 PM   #6
LineArray is offline LineArray  Germany
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I really cannot tell what kind of stuff certain manufacturers
use, but i remember a german manufacturer, who mentioned to use
phenolic resin as doping for his tweeters - which is another
stuff in the list ... but that was decades ago ...

I experimented with all kinds of stuff, rubber solution is not
very durable when exposed to sunlight. And it is rather
aggressive against some kind of plastic parts when it comes
in contact with. Application is not very easy because it
dries fast.

If your meshes are very fine and you want mainly the airtightening
effect i would give a slowly hardening silicone oil a try.
That stuff is rather durable AFAIK and keeps its flexibility over
long time.

The question is whether you want the doping to be the same
material as the glue between surround/cone and surround/basket.
Won't work with silicone.

If your cone is paper and the cloth is cotton based
white wood glue can be used (maybe dilluted) to glue the cloth
to the cone. For the basket side you will need something different.

Maybe 2-Component Epoxy resin mixed with less hardener to stay
a little soft over time, there are also Cyano Acrylate based Gels
which may be ok for that.

Then maybe apply the oil afterwards to the surround.

But hey, there are very much possibilities ...

I would test the doping on the cloth first to see whether
it leads to the desired result. Then test the glues for
the paper and the basket side separately.
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Old 20th June 2010, 02:20 PM   #7
borispm is offline borispm  Hong Kong
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Fostex should be usually in oiled cloth or foam.
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Old 1st January 2018, 05:10 PM   #8
bentoronto is offline bentoronto  Canada
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doped cloth surround?
I thought I'd document my recent experience erroneously substituting a "foam" half-roll surround on a classic (1960) fine 15-inch woofer that had a beautiful accordion cloth surround.

Fs hideously went from about 20 Hz to about 34 Hz*. Here's why.

Classic woofers had light cones and wonderful compact edge-wound coils, say 80 grams, because you want to push light air around with vastly heavier cardboard and copper. Accordion fabric surrounds are very flexible. But they "oil can" by having a motion of their own and are none too forgiving or too linear for large excursions. Half-round surrounds are better in those regards, including the AR half-round fabric experiments.

Today's woofers are expected to be able to play loud - mostly a battle of advertising claims since the loudnesses are unrealistic and also would destroy your hearing in short order. It is common to see cone assemblies weighing half a pound (230 gr) or more in order to handle the load and the related current. Corresponding, they need strong support and so few have really nice low Fs. I'd say that's as doubtful a design as the incandescent bulb of recent memory.

So, if you have a classic fine woofer that you (probably erroneously) want to re-surround, you have to decide if Fs matters to your system. If it does, stick with an accordion surround (or if you can find a certifiably light rolled surround).

B.
*unfortunately, I haven't been able to dig up suitably matching before-and-after performance records; THD with the foam surround seems to be pretty fair, maybe 50dB down from mid-bass up
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Last edited by bentoronto; 1st January 2018 at 05:33 PM.
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Old 1st January 2018, 10:55 PM   #9
phivates is offline phivates  United States
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Location: Willamette Valley
Wharfedale used flat felt back in the 50s-60s, which worked for short excursion drivers. If you are making your own surrounds then some shaping will be called for unless something elastic is used (like felt). Are you sure no foam surrounds are available in the size you need? They can be doubled to increase stiffness.
Or do you have intact surrounds that just need re-doping?

Last edited by phivates; 1st January 2018 at 11:15 PM.
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Old 1st January 2018, 11:44 PM   #10
bentoronto is offline bentoronto  Canada
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doped cloth surround?
Quote:
Originally Posted by phivates View Post
Wharfedale used flat felt back in the 50s-60s...
Many thanks for your information. But my interests just now are experimental* rather than vintage restoration. My purpose was just to ensure Google archives the data (and musings) posted above so others can find it.

B.
*I'm trying to figure out how to attach an accelerometer to an absolutely rigid but removable "dust cap" structure. And how to attach a shielded lead to it. And how to get that lead out from the cone assembly to the motional feedback circuit.
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