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Old 14th June 2010, 05:36 PM   #1
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Default Re-foam question I haven't seen asked/answered yet... ??

I've successfully re-foamed a few drivers. One issue that's occurred to me - and I haven't found it discussed - is how do you ensure the new foam surround has the same compliance as the old foam?

It's easy to find a source of a foam that fits a driver perfectly. However, in my experience, the new foam is always stiffer than the old, causing a rise in free-air resonance.

I do realize that old rotting foam may be extremely compliant. But I do have a few woofers for which I measured free-air resonance when they were new (in the 1970s/1980s) and after re-foaming, the resonance is noticeably higher.

Anyone with ideas on how to find foams with the right characteristics? I know that correct foams for classic JBLs, Altecs, Advents, are easily found... but such is not the case for CTS or Eminence woofers which may have been used in high-end "brand name" systems...
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Old 14th June 2010, 05:51 PM   #2
badman is offline badman  United States
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You could massage it, to soften it up, like is done with leather. Other than that, you're SOL I do believe, unless there is a matched foam from the manuf, or you want to try a chemical reduction process.
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Old 15th June 2010, 03:42 AM   #3
kevinkr is offline kevinkr  United States
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Is this even after running the driver for some hours in order to break in the new surround? Every cone driver I have bought in the past few years has required at least 20hrs of run in time to achieve anything close to the ts parameters you would expect to see.
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Old 15th June 2010, 04:52 AM   #4
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Yes, I've run them for hours with no noticeable change.

In one case, I ran a woofer for 48 hours, driving it in free air just below its resonant frequency. Because of the high impedance near resonance and the low driving voltage (I think it was 2Vrms) very little power actually was delivered into the voice coil, but there was a fair amount of excursion. Even after this "break in," the resonant frequency was unchanged.

The new foams just feel stiffer and thicker than the old ones. I know that "feel" is subjective, but intuition/experience can be helpful. Are newer foams made of different materials which are considered more durable (but are less compliant)?

I've never seen a discussion on this topic, but I would think it affects a very large percentage of re-foam projects.
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Old 15th June 2010, 10:53 AM   #5
Ron E is offline Ron E  United States
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changes in compliance don't change box performance much - I wouldn't be worried about T/S parameter shifts, but changes in the top end rolloff of anything that is used up near breakup is a more troublesome issue. The surround also terminates the cone resonances and could cause cancellation or reinforcement if its mechanical impedance is different.
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Old 23rd June 2010, 01:43 AM   #6
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I'm planning on using these woofers well below the frequency where any break-up might happen, so the termination issues you mentioned won't be a problem in this particular case.

On other re-foam projects, I've noticed a shift (upward) of as much as 20% in free air resonance with new foam vs. its original measured value.

What I've started to do is order replacement foams from various sources and measure their actual effect on my test woofer. I haven't been able to figure out any other way to determine which new foams will come closest to providing the original parameters.
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Old 23rd June 2010, 12:38 PM   #7
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Read thru this thread here Compliance of replacement foam surrounds - The Classic Speaker Pages Discussion Forums
and in particular, post #40 which my rant on an established speaker manufacturer industry test method for measuring compliance. However, that data is kept private. Attempts on my part to free up the data fell on deaf ears.

You just have to trust your suppliers......
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Old 23rd June 2010, 06:00 PM   #8
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Wow, great reference. Thanks.

FWIW, the way I'm testing the new surrounds is this...

First, I clean off all the old surround.

Then, (don't laugh) I put double-stick tape on the edge of the cone where the surround will go. I press the new foam (I mean the new surround) on there until it sticks.

Third, I clamp the outer edge of the surround to the basket using spring clamps like you'd get at any office supply store for holding together a bunch of papers. If I have an intact from gasket from the speaker, I put that on first, and then clamp. If you feel the voice coil rubbing, it's easy to remove the clamps and re-position the outer edge of the foam until everything is aligned correctly.

At this point, you measure the free-air resonance using the method of your choice.

The only problem I've found with this is that the double-stick tape is not very sticky and doesn't adhere for very long. Once you have this thing all set up, you need to do your measurements right then.

When done, you can take everything apart, and all the components are still good - even the foam.

Clearly, this is not the method for evaluating the sound or performance above the frequency where cone break-up occurs.

Last edited by Robert1954; 23rd June 2010 at 06:15 PM. Reason: clarification and additional thing to mention
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Old 23rd June 2010, 06:56 PM   #9
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For those interested, PM me and I'll email you the test method and information on the new, modern test instrument used.



Quote:
Originally Posted by Robert1954 View Post
Wow, great reference. Thanks.

FWIW, the way I'm testing the new surrounds is this...

First, I clean off all the old surround.

Then, (don't laugh) I put double-stick tape on the edge of the cone where the surround will go. I press the new foam (I mean the new surround) on there until it sticks.

Third, I clamp the outer edge of the surround to the basket using spring clamps like you'd get at any office supply store for holding together a bunch of papers. If I have an intact from gasket from the speaker, I put that on first, and then clamp. If you feel the voice coil rubbing, it's easy to remove the clamps and re-position the outer edge of the foam until everything is aligned correctly.

At this point, you measure the free-air resonance using the method of your choice.

The only problem I've found with this is that the double-stick tape is not very sticky and doesn't adhere for very long. Once you have this thing all set up, you need to do your measurements right then.

When done, you can take everything apart, and all the components are still good - even the foam.

Clearly, this is not the method for evaluating the sound or performance above the frequency where cone break-up occurs.
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Old 23rd June 2010, 07:24 PM   #10
limono is offline limono  United States
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I'm not sure if I stay on the topic but I'm on the stage 2 of my JBL LE 8 repair saga. I've got a pair of refoamed early black cone drivers. Unfortunatelly the drivers VC's were partially shorted (probably thanks to original owner trying to play music trough the speakers with rotten surrounds and VC's rubbing the magnet gap assy) They measured 1.3-1.5 Ohm instead 5.4 Ohm. Ater entertaining recone options ($202 each from JBL -ouch)and trying to find a nice pair for reasonable money (nope!) I decided to try my old friends trick.
I heated and separated lower suspension from the basket and took the cone assy out . I filled the cup with shellac enough to submerge VC in it when the rest of cone sits on the cup and connected the wires to car batter charger (2 amps) in short intervals in order to heat them up enough to separate from each other and let the hot shellac penetrate and insulate the coil.This primitive method worked OK and now I have 5.4 ohms reading on the coils. I ordered a suitable surrounds and now comes the sticky bit .The alu dust cover part is integral part of the driver and I'd like to keep it intact. It makes shimming the coil from inside impossible. How would you aproach putting it all together ?.Start with glueing the surround to the cone then position it in the basket, clamp it .try if it is not rubbing (play some test signal -music generator) than slowly glue in lower supension section after section lifting it a little and holding the cone in place.Thats my plan but I did not attempt to work on the speakers for years . Regards, L
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