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Foam for baffle diffraction

I would like to start trying some foam on baffles to reduce edge diffraction.  I plan on "inlaying" the foam to make it flat with the rest of the baffle. What kind should I use?
I have seen thick felts, open-cell foam, closed-cell foam, and batting. I have no idea where to start.
Thanks for the help.


  • Like this?
    Hit the local hardware store for some wool and foam for filling in gaps.  Wondered around the fabric section of wallyworld and settled on some super fuzzy and thick yarn.  They also had some cloth+batting that looked promising.  The thing with padded fabric is that some of it might actually look good on a speaker.  The padding does make a difference to my ear and measured.                 
  • Nice!
    I might need to go to the store and pick up some thick felt or possibly check the quilting section for some thicker material.
  • I got some 1/4 inch felt with self-stick adhesive on one side from McMaster-Carr. IIRC, they had a variety of thicknesses and densities available.  Although these methods may reduce face diffraction, it's my feeling that a sharp 90 degree edge on the baffle will still produce the well known baffle diffraction effects. 
    Don, Donno, or "Hey you" all work for me, But never "Mr Johnson"
  • While we're on the subject of diffraction/absorption etc. 
  • David Ralph did a lot of testing of the effects of different types of felt shapes.  See the Diffraction section on his website.  LINK
  • Nice site.  Added to favs.  
  • +1 McMaster Carr has got the biggest selection of wool felts. Based on the adsorption coefficients I would use only wool.
     John H, thanks to JP I did get that email
  • Thanks for the great info. I'll order some of the wool and start experimenting. 
  • @kenrhodes - not sure what you'll end up with, but that laser engraver you pointed me to cuts the charcoal open cell foam like a dream.  Takes a while with the 2.5W laser we got (1" thick moves along at about 1.5mm/s), but it's a perfect cut. 
  • Dan-It's like you read my mind.  B)
  • I posted a similar question about 6 years ago and received many differing responses.  So I did a small experiment where I router out a 1/2" deep cavity around a RS28A flange and inserted a 1/2" thick felt ring that totally filled that cavity and was perfectly flush with the baffle.  It actually looked really cool but the measured results were much worse than the typical flat baffle.  I made a third baffle where I just stuck that same ring of felt on the surface of the baffle (no cavity) sitting 1/2" proud of the baffle.  It wasn't very attractive to look at but the reduction in tweeter diffraction was very significant.  If I were the kind of guy that liked to put grills on my speakers I would make them all a 1/2" thick and apply 1/2" thick surface mounted felt around the tweeter.  I wasn't able to or interested in doing subjective A/B listening tests.  I just did on axis measurement.
  • When you run fingers across the lands between the cavities encircling the tweeter (pictured above)  the mdf makes an audible clanking or ringing sound.  Without the cavity being filled I'd imagine that clank/ring might be detrimental to the tweeter's sound. 
  • I used adhesive backed felt bobs on a baffle once. Ugly, but very effective. 
    I have a signature.
  •  I've said it before, speakers want to be ugly.
  • Back in the day RJB made a 3way tower with a felt insert around the mid and tweeter that I thought looked pretty attractive.

    I'm not deaf, I'm just not listening.
  • Oh yea.  Looks much better than my worm tracks around the tweeter. 
  • From the write-up it looks like the felt is stuck on the baffle.

    I decided to cut out a thin piece of black felt and applied it to the baffle. The felt covers most of the tweeter flange and part of the midrange frame.

    Stuck on the baffle would be good, but what about the baffle being recessed, do that the felt is flush? Something like the Wilson's? There seems to two layers of felt, but they seem to be flush at the edge of the baffle, would the felt reduce or nullify the diffraction due to the tweeter being recessed?

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