"A few decades ago I thought about how most of my better speakers had rubber edges and
wasn't this great since I would not have to refoam them. Well ....
I want to warn that most drivers with "rubber" edges harden, grossly shifting the resonance of
the driver. Especially Vifa drivers but many, many others.
I have 4 Vifa mids with rubber edges, all of them show Fs being double what it should be which
indicates that the edge is about 4 times stiffer. 2 Vifa woofers. 2 PSB woofers. 2 Vandersteen
2Ci active couplers and on and on.
Also note that glues often fail, magnets fall off, spider joints break, etc. some of these issues
can be fixed.
And of course ferro fluid hardens.
Speakers that I've measured:
Vandersteen 2Ci Active coupler edge hardened, P21 Vifa is the type that hardens. Some 2C series
had foam edges on the woofer and mid that must be refoamed.
Dahlquist DQ20 rubber edge on Vifa mid hardens. Note that DQ10 has a fabric edge on the lower mid
and should not have this problem.
PSB Stratus Gold Main 10" woofer rubber edge (the same part as used in 2Ci Active coupler) Fs showed
double what it should be. Midrange is a Vifa and is also probably hard.
Spica TC-50 These are a mess, Audax woofer edges are hard, probably shrunk causing spider sag, AND
of 6 woofers that I tested all of them had their HF peak significantly shifted and no longer matching the
factory "tuned" crossover. All showed spider sag.
Celestion SL6 Woofer edges harden and seem to cause spider sag, no current fix for this. The Si version
uses 2 different types of "rubber" for the edge and do not seem as bad. Half of that edge probably does
harden.
2 woofers that I've measured as good are the KEF B139, and the SEAS in the Dyna A25 which probably
have butyl rubber edges, however this was over a decade ago so I should measure them again. They are
probably okay because butyl rubber lasts a very long time. Some versions of the KEF B200 have been
reported as having problems with the edge but I've not measured any. I have a pair of the newer KEF B200G
that do not show significant spider sag but I've not yet measured Fs.
We're probably best off with speakers with foam edges but they often need to be replaced every 10 years or so.
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On the Spendor group, the brake fluid has been working for years, with annual 'touch-up'.
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Maybe a silicone lubricant like WD-40 would work?!
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I’ve got a pair of original TC-50s that I haven’t fired up in at least 10 years, but I can’t bear the thought of disposing of them. I suppose they’ll have the same issues as the TC-50 woofers you examined. Is there a driver rebuilding company that you would recommend?
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I would suggest that speaker surround degradation is more often a case of polymer oxidation which destroys the bonds rather than their crosslinking.
Think of it like tires disintegrating with age.
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The brake fluid is for rubber surrounds that have hardened. Foams stuff disintegrates, plastic ones crack.
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Frank, can you explain this a bit more and is there any chemical that you suggest?
Brake fluid works by the way, and Wintergreen oil works but doesn't last.
I've tried WD40 and it helps but not by much, I believe that it is the solvent in it that makes the difference.
My dad was a polymer expert and his company designed and constructed many rubber plants for
several of the largest tire companies. He like to lecture, even at home, and I asked him about rubber
edges hardening and he didn't have a good answer or a lecture so, I would guess, that it is not
I would suggest that speaker surround degradation is more often a case of polymer oxidation which destroys the bonds rather than their crosslinking.
Think of it like tires disintegrating with age.
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I believe that the "rubber" surrounds causing the most trouble are actually "rubberized" plastic if there
is such a thing, vinyl rubber. Perhaps like a garden hose. As I said butyl rubber edges last at least
30-40 years if not more. Butyl is far more stable.
I measured the woofers in my PSB stratus Golds in the 1990s and they were already close to 4X harder
than originally. I spoke on the phone with Paul S. Barton (PSB) to find out the spec for the woofer and if
they knew about the issue.
The brake fluid is for rubber surrounds that have hardened. Foams stuff disintegrates, plastic ones crack.
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I've not yet tried brake fluid on the TC-50 woofers, it will likely soften them and if the rest
position goes back to center then they might be usable. No one can rebuild them. not
even Miller Sound. I would not throw them away, but rather replace with a Dayton 7"
woofer and retune the notch, recap the XO.
There were other candidates for replacement woofers but these are NLA:
Anarchy
Meniscus 6502
There's a Scan Speak that might work but >$200 each
My latest idea is to find an old Boston Acoustics (or polk) 7" 4 ohm designed for sealed box
and refoam them. No notch needed, just remove it. Redesign the crossover as needed.
I’ve got a pair of original TC-50s that I haven’t fired up in at least 10 years, but I can’t bear the thought of disposing of them. I suppose they’ll have the same issues as the TC-50 woofers you examined. Is there a driver rebuilding company that you would recommend?
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