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Thread: foam or felt in the cover

  1. #1
    Senior Member louped garouv's Avatar
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    foam or felt in the cover

    so, I know that JBL used a bit of foam in the back cover
    of the 244* drivers, which had a tendency to degrade over time...

    and Altec used felt in the back cover of the 802 series drivers...


    if replacing foam in the JBL drivers,
    would it be OK to use felt, as it does not degrade
    like the foam would...


    pros/cons of each?

    other opinions?

  2. #2
    Obsolete
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    Having replaced tons of that nasty foam with more foam I'm going to start looking for some felt. I'm finally fed up with the foam.

  3. #3
    RE: Member when? subwoof's Avatar
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    mosaic

    I found a small sheet of the multi-colored mosaic carpet underlayment at a fabric store - it's thickness was almost the same as the 244X foam and it worked well but haven't found any lately..I have also seen it used inside jackets.

    I remember that altec used it for some of the 800 series drivers. Have a boatload of drivers in storage and it's time I found some more of it. When I do I will post the info.

    sub

  4. #4
    Senior Member grumpy's Avatar
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    ... I've used the same material..., time will tell if it was a good idea.
    I'd be interested to hear how your 800-series Altec's have fared.

  5. #5
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    Mc-master carr sells F11, F13, etc. felt (you want the heavy wool content) that is appropriate for the task. Members in soCal are welcome to pick up a small piece from me if it's more convenient, I've got enough to share. You can use a lot of this stuff though, so if you want more than a couple driver's worth you should just order some. It makes great baffle treatment (absorbtion before the diffractive cabinet edges).

  6. #6
    Senior Member grumpy's Avatar
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    Good to have a source (or two ). Thanks for the reference and kind offer!

  7. #7
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    Probably something like this

    http://www.mcmaster.com/#f13-felt/=49a97r

  8. #8
    Senior Member grumpy's Avatar
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    Yep. This URL specifically has the self-adhesive backing, 1/4" thickness, and
    reasonable storage requirements (12"x12" square). $7 + S/H/Tax...

    http://www.mcmaster.com/#87415k53/=49agru

  9. #9
    RIP 2011 Zilch's Avatar
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    Some LE85s I've opened have had felt in them instead of foam; I'm suggesting that there is precedent in the legacy....

  10. #10
    RE: Member when? subwoof's Avatar
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    peeled away

    I found that the felt is *under* the foam on those drivers and the sister pro models. After the foam goos away the adhesive backing peels off quite easy.

    I make it a point to remove this for storage..

    sub

  11. #11
    Senior Member 4343's Avatar
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    Exclamation Yep, felt under foam. See it peeking out in first pic...

    Quote Originally Posted by subwoof View Post
    I found that the felt is *under* the foam on those drivers and the sister pro models. After the foam goes away the adhesive backing peels off quite easy.

    I make it a point to remove this for storage..

    sub
    I used some foam I had on hand a while back for an old, corroded 2470. The original diaphragm had opened due to corrosion as well. I had a 2425H dia on hand, so in it went. I put the driver into service and didn't think about it till today when I saw this thread. When I opened it to see if there was felt, I noticed that the foam I used (an egg crate pattern soft open cell type) had been pressed back by the diaphragm forming a ring indentation in the foam! I had put too thick of a foam in!


    The interesting thing about this driver is that the center bump of the foam was actually pressing on the diaphragm as you might be able to see in my pics. For years I've heard how Ti sounds harsh, needs to be damped, etc, but ever since I changed a few of my drivers I did not think it sounded that much worse than the Aluminium I am still using. Today I found out why it did not bother me... Once I put thinner foam in I started hearing the harshness everyone has been talking about! Granted I did use a Constant Directivity curve on a driver mounted on a 2312, and cranked it up to SR level, but as soon as I put the thicker egg crate foam back the harshness mellowed out.

    I checked my 2420s and they have the same foam pressing against the diaphragms, no wonder I like the 2425H dias in them! I feel it's a similar idea to the damping foam under some of the Ti dome tweeters, and it seems to work for me... If anyone wants a piece of this foam to try it themselves, I have enough to make about 4-6 more pieces.
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  12. #12
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    Dampening qualities of material

    This is a good engineering project for someone with the instrumentation. There are endless materials all of which have different absorbing characteristics (car companies do an enormous amount of this sort of thing in their "N&V" labs (noise and vibration). Has JBL done this somewhere in the past? Greg? But, in any case, it will require some testing. For what it's worth, I recently replaced the foam in my 375 drivers which I purchased new in 1979 with the factory foam which is still available at your local JBL pro shop and it fit perfectly and the drivers sound like new. That's good enough for me. The originals lasted 30 years in a home "hi fi" environment - perhaps the degradation is accelerated in pro audio applications (cold, hot, wet, corrosive, et al). But, the comments below re the dampening effect of physical contact with a foam insert would certainly be true; unavoidable. But I'd guess there is also a significant effect on response which is likely a large part of why they sounded less "harsh." Gotta take some data.



    Quote Originally Posted by 4343 View Post
    I used some foam I had on hand a while back for an old, corroded 2470. The original diaphragm had opened due to corrosion as well. I had a 2425H dia on hand, so in it went. I put the driver into service and didn't think about it till today when I saw this thread. When I opened it to see if there was felt, I noticed that the foam I used (an egg crate pattern soft open cell type) had been pressed back by the diaphragm forming a ring indentation in the foam! I had put too thick of a foam in!


    The interesting thing about this driver is that the center bump of the foam was actually pressing on the diaphragm as you might be able to see in my pics. For years I've heard how Ti sounds harsh, needs to be damped, etc, but ever since I changed a few of my drivers I did not think it sounded that much worse than the Aluminium I am still using. Today I found out why it did not bother me... Once I put thinner foam in I started hearing the harshness everyone has been talking about! Granted I did use a Constant Directivity curve on a driver mounted on a 2312, and cranked it up to SR level, but as soon as I put the thicker egg crate foam back the harshness mellowed out.

    I checked my 2420s and they have the same foam pressing against the diaphragms, no wonder I like the 2425H dias in them! I feel it's a similar idea to the damping foam under some of the Ti dome tweeters, and it seems to work for me... If anyone wants a piece of this foam to try it themselves, I have enough to make about 4-6 more pieces.

  13. #13
    Senior Member sonofagun's Avatar
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    If that foam is similiar to that used for speaker grilles (open cell), I can provide non-disintegrating foam replacements if someone will supply the specs (dimensions).

  14. #14
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    FYI, some Altec 800 series compression drivers had loading caps (plastic covers that fit over the back of the diaphragms) from the factory and others did not. For the loading cap drivers, no felt was used on the back covers.

    Felt, however, was used on covers of those drivers that did not ship with loading caps.

    Some beleive that many earlier drivers did not use loading caps and that later when amps became more powerful, loading caps were more often used. Apparently, loading caps were more often used in commercial applications including cinemas and SR.

    Felt can be found for purchase in craft stores.

  15. #15
    Senior Member herki the cat's Avatar
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    Foam & Drivers

    Quote Originally Posted by 4313B View Post
    Having replaced tons of that nasty foam with more foam I'm going to start looking for some felt. I'm finally fed up with the foam.
    Been there !!___ In 1972 a co-worker in RCA Camden bought a huge roll of "Sound Coat Inc., Acoustic Foam, 2 " by 6 ft by 20 ft costing him $400, which we were going to share. We never found time to stuff this junk-goop into a kit, of his__ two, & my__ four, RCA LC9-A horns.

    Thirty years later in my friends garage, this huge roll of "sticky muck" collapsedto to the floor & the poor guy's hair turned White! We were gratefull this sticky melt-down did not occur around our priceless drivers and the inside surfaces of our horn enclosures..

    Foam is worse than you think. Most foams are made from polyurethane which ultimately reverts to a liquid sticky state similar to the parent polyurethane paint finish-genere . The sticky foam crumbs disloge & do move around__ not a good idea around exquisite, priceless LE 175, LE 85, & the 2420 professional driver aluminum diaphragms, which are no longer available from JBL. For the 375 & 2440 drivers, the replacement diaphragms cost some $250 each if you are lucky to find stock.

    Felt damping against titanium diaphragms in the 2425 / 2426 drivers sounds like a better choice__ By all means, wool felt is ideal & it is available from professional Artists Etching-Press Supply Sellers & other industrial sources.
    ------------------------------------------------------
    cheers, herky the cat
    Last edited by herki the cat; 11-06-2009 at 12:49 PM. Reason: Text Clean up

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