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 Attachment(s)
Yep, felt under foam. See it peeking out in first pic...
Quote:
Originally Posted by
subwoof
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!:o:
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.
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
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!:o:
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.