Why replace the foam on speakers?

January 27, 2013 3:55 PM

I have an old (30+ years) pair of speakers, and the foam rubber surrounding the woofers is cracking and falling off. Numerous DIY websites explain how to replace this foam, but none seem to answer the most important question: why would you bother doing this? The speakers still make sound.

I have a pair of (I think) 1970s-vintage Bose 301 Series 1 speakers obtained via a particularly successful dumpster diving trip about a decade ago. I listen to music through them every day, and they sound...fine, I guess. I don't have another pair of speakers to compare them to, but music reliably comes out every time I push play.

When you look at the woofers, though, it's obvious that the surrounds are in really rough shape. They are cracked almost the whole way around, and in numerous places the foam has actually fallen off, leaving a gaping hole. If you poke the rubber lightly with your finger, it kind of crumbles under the pressure. That can't be good, right?

So, the question is: why should I bother re-foaming these speakers? Am I risking damage by not replacing them? Will my sound quality improve? I'm happy to take on the project, but I'd like to know what I'm actually accomplishing before I start.

posted by wondercow to Technology (3 answers total)

Best answer: The foam holds the cone in-place and allows the cone to travel accurately. Without the foam, the cone flops-around when driven, resulting in muddy and harsh sound. Re-foam the drivers or replace them completely. I opted for the complete replacement when the foam on my JBL L-100t's fell to dust.

One caveat...Try to find a factory re-foam kit. Generic kits sort of work, but aren't an exact match.
posted by Thorzdad at 4:07 PM on January 27, 2013

Best answer: If they still work, I'd leave them. However, it's not typical for speakers to work satisfactorily when the rubber around the perimeter of the cone is gone. The cone in loudspeakers typically are suspended in two places. The perimeter of the cone to the frame (the surround), and the area around the coil, called the spider. This allows the cone to move in one direction. Back and forth in relation to the magnet. If the surround (the foam perimeter of your speaker) no longer supports the wide end of the cone, then the cone can rock side to side, using the remaining attachment point around the coil (the spider) as a pivot. If it gets bad enough, the coil can make contact with the magnet, and the cone can buzz against the frame. Either way, it would likely be very noticeable in terms of yuckky sounds. It's possible this kind of distortion can do damage to the remaining speaker. If it isn't, it sounds like the remaining suspension (the spider) is rigid enough to keep this from happening.

Being lazy, I'd probably leave it if it caused no problem.
posted by 2N2222 at 4:10 PM on January 27, 2013

Yes, you should replace them. Once the cone starts getting wobbly, it will short out in the sides of the magnet. This happened with my Pioneer speakers, and they kinda sorta worked for a little while until I started hearing clicks and buzzes, which I didn't think was going to be good for my amplifier. I replaced them with more or less equivalent drivers and the improvement in sound was tremendous. Not sure if it was because they had worn out, or if speaker technology had significantly improved in the 20 years since they were manufactured.
posted by gjc at 5:11 PM on January 27, 2013