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January 28, 2012

Fixing Old Speakers – The Common Woofer Foam Rot Problem

Audio Research freebie speakers

Audio Research freebie speakers


Got an old pair of speakers that don’t bark as well at the low end as they did in their younger days? You might be suffering from the dreaded woofer foam rot. Recently, I had been hunting for a good, cheap pair of speakers to partner with my newly repaired Hitachi receiver. I luckily found some free on Craigslist; someone just 2 blocks from me was giving away a great pair of 4-way Audio Research speakers with massive 12″ woofers, probably 1980’s vintage. They were “great”, that is, except that the suspension foam surrounding the woofers had almost completely decayed away.
Foam Rot

Woofer Foam Rot


This is the well-known “foam rot” problem seen in woofers and other high-compliance drivers of similar age. A few decades ago, speaker manufacturers started using a glued-on ring of shaped foam in place of the concentric corrugated cardboard surround on hifi speaker woofers, presumably to reduce distortion, especially at high excursion amplitudes. Unfortunately, the use of this foam has a recomplicating effect; it tends to decay away in 20 or so years, rendering the woofers useless. (The foam is apparently far from stable, which is good reason also to wear gloves when doing this repair; I don’t know the composition or the decay products.) Fortunately, there is an easy solution; there are many vendors of new, precise-fitting woofer foam, and the repair can be done quickly.
First, you need to seek out replacement shaped foam ring to fit your specific woofers. The key is to match the various diameters involved. From inside to outside, these are the inner diameter where the overlap of the foam and woofer cone ends, the outer woofer cone diameter where typically the curved topology of the foam (the actual compliant suspension begins), then the outer diameter of that suspension adjacent to the woofer metal frame, and finally the outer diameter of the foam. Those diameters, especially the inner two, all need to match your specific woofers to better than 0.25mm, I’d estimate. There are many vendors of such foam to be found on sites such as eBay and Amazon, as well as electronic parts suppliers such as Parts Express. Some vendors supply also glue with the foam in kits, but I have a good supply of glues, so I elected to buy just the foam. I found a reputable vendor on eBay, “speakerguy2002”, and was happy with the parts I received ($8.95 for the pair).
The first step in the repair is to fully remove the woofer from the speaker enclosure. This requires moving the mounting screws and disconnecting (possibly desoldering) the connectors from the woofer. In my case, I also needed to pry off the “pretty ring” covering up the screw heads. Others doing this repair have needed to use a hair dryer to weaken the glue on the pretty ring but in my case it pulled off easily. While you’ve got the innards of the speaker enclosure exposed, you can take this opportunity to check for problems like leaky electrolytic capacitors used in crossovers (mine had none).
Removal of the "Pretty Ring"

Removal of the "Pretty Ring"


Woofer connections

Woofer connections

The next step is to clean away the old foam. I avoided using any solvents at this stage, and found that was able to just gently but thoroughly rub away the old foam with my fingers (wearing protective plastic gloves to keep the gummy old foam off my hands). I vacuumed the area frequently during this stage to remove loose foam bits; there were a lot of those. The cleaning job doesn’t need to be perfect but you need to remove the old foam to reveal a surface for gluing to. You can see a partially cleaned cone in the following intermediate photo.

Cleanup Closeup

Cleanup Closeup


For gluing the foam to the cone, I used Elmer’s safety glue, and for gluing the foam to the metal woofer frame, I used a thin contact cement. I picked contact cement for the gluing to the metal because I had found in tests that Elmer’s did not adhere to the painted metal well, whereas the contact cement worked great. The contact cement I used was a thin, transparent one for photo mounting, but most other general purpose contact cements should be fine. You just don’t want a really thick one, so as to have a fairly uniform seal. Work with them in a well-ventilated area due to the vapours that they give off, and don’t inhale directly over the glue. I applied glue to the cone first and attached the foam to that, then moved on to the other woofer of the pair while the first glue on the first woofer set to some degree. Next, I did the contact cement gluing of foam to the frame. Prior to doing that, I had checked that the cone + voice coil traveled freely up and down in its suspension without interference, by applying balanced finger pressure and also by applying a low voltage supply from a variable DC supply. Other folks have often gone to the trouble to remove the dust cap that is typically centred on the woofer to work on the voicecoil alignment, but my tests showed the alignment to be OK with no interference in the cone movement; this was a quick job on low end freebie speakers, so I didn’t bother with that alignment effort in this case. Worth looking into, but in this case the end result was fine without it.
Cone gluing

Cone gluing


During the contact cement stage, I glued diametrically opposite points and worked around the circumference, so as to maintain balance of the cone and not mess up the voice coil alignment. I found some cheap spring clamps that I’ve used in many other projects helpful for holding the foam in place on the frame while the glue set overnight. I shuffled the clamps around the perimeter after initial contact to make sure of a fully contacted seal all the way around.
Clamps for holding glued foam onto frame

Clamps for holding glued foam onto frame


I was happy with the end product. The speakers sound great paired with my free Hitachi receiver from this earlier post, although I don’t generally don’t play them loud enough to offend our large 4-footed woofer, the Gimli Bear.
Our 4-footed Woofer, The GimliBear

Our 4-footed Woofer, The GimliBear


Make sure that you wait 24h for the glues to dry before any heavy duty use of the speakers. Happy woofing!
The Finished Product - Happy Woofers!

The Finished Product - Happy Woofers!

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