Speaker Dust Caps and Refoaming Speakers

Hey, folks.

so, I'm getting ready to refoam a pair of 12' Eminence speakers. These were made in 1980 for a set of Acoustic Control Corp speakers, model 626.

This has been discussed here and other places, but thoughts on to shim or not shim during a refoam? My gut is telling me to shim. Also, called a guy here in Austin, TX who fixes speakers and his take was always to remove the dust caps and shim when doing a refoam job.

Speaking of dust caps, these Eminence were fitted with felt caps that measure about 4 1/4". I'm having a hard time finding this exact cap online, so was wondering what people think about substituting paper caps for felt. Is there much of a difference in sound? These speakers have non-vented magnets and I've heard that felt is generally used for this type.

Any insight or opinions would be useful!

 

I've done a few dozen refoams over the years and never shimmed and the results were perfect. BUT....most all those had fairly stiff cones and spiders. If I encountered a thin flimsy cone/and or a really flimsy spider I'd probably opt to shim as it would be easy to distort the cone and/or mess up the alignment of the voice coil.

As far as substituting caps, I'd be concerned about the weight first, then the size and height of cap. Any large deviations from these could change the performance aspects of the driver.

As you mentioned also being non vented, the felt cap would provide some degree of venting helping to keep the voice coil cool. A solid paper cap may not give enough cooling. That I don't know much about though, just a guess.

 

Thanks, Guild. Very useful info! Just ordered the new foams today so hope to tackle this later this week.

 

I had a guy redo a woofer and he removed the cap, replaced suround/shimmed, and then stuck the dust cap back on. Might not always work, but he was willing to try and it worked fine.

 

I think I'm going to try this without using shims. The cones are solid and the felt dust caps are in perfect shape and I'm just not wanting to remove them. There are some good videos out there that show good centering techniques without using shims. I think I'm going to try to glue the inner part of the foam first, let it set over night, and then the next day glue the outer part and center the voice coil. I can always use clothes pins or something around the outer diameter to hold it in place once it's centered and drying. We'll see how this goes...

 

I did a pair of 15" woofers from some old Radio Shack speakers years ago. It was really easy. I shimmed and it came out perfect. Once you do one, it's really not that intimidating or difficult.

 

You could play a 30hz test tone through the speaker when refoaming as well. There are many websites out there with them, but here's one that you could use: http://www.eminent-tech.com/music/multimediatest.html

 

Thanks, Rodant. Great idea.

 

Not sure what value there is in this, it actually may damage the speaker if turned up to far. The test is, is there ANY, even faint sound/feel of voice coil rubbing once refoamed? If there is, you need to redo it, the voice coil is not centered.

 

^^^True. That tone will let you know better than eyeballing it though. If you hear any distortion, it isn't centered. That tone isn't a rock song - don't turn it up too loud

 

When I've had the surround replaced on a few of my speakers, it always seems to me that the shimming is not just to center the voice coil. It's also to allow you to set the proper height of the cone while "at rest". I once saw a friend's speakers after he had the surrounds replaced and the cone was almost bottomed out and the surround foam was stretched to it's limit when the speaker was at rest. It sounded terrible. I told him he needed another re-foam. He did and it sounded much better when he set the height before gluing the surround.

 

I guess it could be the case that some spiders sag in the middle, and a level height needs to be maintained while gluing the surrounds. I could see shims being useful in that case for sure; not just for centering.

The more I read about this, the more I realize how divisive this issue is. Some folks say "absolutely shim at all costs", and others say the exact opposite.

 

also, most folks selling foam surrounds and kits say shimming is not necessary. Only a couple say that shimming is a must. Most instructional videos out there bypass the shimming step and show various methods for centering, from touching to test tones.

 

If the spider is that weak that that cone sags in I'm no so sure the speaker is worth repairing. It will most likely bottom out on strong bass notes with a spider that weak.

 

I would tend to agree with you. The ones that I'm replacing fortunately don't sag. They appear level, so hopefully these guys will come back to life with new foams.

 

I have been doing refoaming both ways. I greatly prefer shimming, it´s so much easier to get everything right that way. No need for new caps, it´s rather easy usually to cut them out, e.g. with a razor blade, and then just glue them back on.

 

After refoaming a speaker without shimming and not getting it quite right...and then refoaming with shimming and having perfect results...I will always shim in the future.

 

I've done refoaming twice. And I think shims are the way to go. The last thing you want to hear after all that work , is the voice coil scuffing due to a slight off center, which is pretty easy to happen. You may not even hear it at low/moderate volumes but you might at higher levels, with more excursion.

I've done woofers in 1st gen Paradigm Atoms and larger woofers in Bose 301's. I also removed dustcaps the first time but cut them the second time in an X pattern, then ran a bead of glue over to reseal them.

 

I decided to go with shims. I carefully removed the dust caps last night by cutting around the base of them with my x-acto knife. I left a small "hinge" so the caps aren't completely detached. I'm pretty sure I can glue these back down quite well when finished. Inside the voice coil, I marked the level of the cone at rest. I made a bunch of shims out of paper and will use those to hold the cone centered and at the correct height. Also, gonna go with Aleene's tacky glue, as quite a few folks recommend it over the white glue included in so many kits (which is probably just Elmer's...).

New foams arrive today so getting near the end here. I'll post an update.

Thanks for all the great advice, guys!

 

Just wanted to follow up with an update: The refoaming was a huge success. I shimmed using strips of paper I cut in varying thickness. I cut the dustcaps mostly off leaving a small hinge. This made them really easy to glue back down in exactly the same place they were. Cones are perfectly centered; not even a hint of rubbing.

These Eminence woofers have a really nice bass response. I have to say, these speakers overall just shine. So, so nice. Been fiddling with the mids/highs level controls this evening and getting them dialed in for my living room.

Acoustic control corp knew how to make speakers. I can't believe how good of shape these are in being from 1980.

 

I've only re-foamed 6 pairs of speakers and shims work great. One set of foams for Boston A150 woofers included a disc with about 3 minutes of 30Hz test tone which worked well too and was simpler. Using a 1.5 volt battery is supposed to work also but I never tried it.

One pair of woofer that were, I hate to say, impossible for me to foam were from Wharfedale Dovedale III's. The original rubber surrounds BTW were not impervious to deterioration as they had hardened to the consistency of dried clay. Underneath the dust caps were what I thought were aluminum "damping rings". Thinking they were just that, a ring glued onto the cone's apex, I proceeded removing them using TV & Radio Solvent to release the glue. As this process proceeded I discovered this "damping ring" was not a ring at all but a cylinder that continued well inside the the voice coil form. By the time I realized this I extracted it about a half inch inch then chickened out and took the woofers in for professional help.

 

Shim and reuse the old dust caps. I use 100% acetone (nail polish remover) Brush on with an model paint brush and let soak. apply a few times and let it work it's magic.