I'm new to speaker building, and as an experiment wanted to make my own foam surrounds for an old pair of 4" drivers from a Koss M85-Plus (exact commercial replacement surrounds not found). I was not concerned with exactness; these were cheap and I wanted to see how different homemades would be from the originals.
My local craft stores only carry 2mm foam which I feared might be too thick/heavy, but I found 1mm EVA foam online. (
1mm EVA CRAFT FOAM SHEETS 12"X18" 15 pcs, choose or write in your choice. 762668731008 | eBay)
I happened to already have a router and a 1/2" diameter core box bit (which are cheap). I also happened to have several sizes of clear tubing/hose (available at most home stores) and found a 3/8" size to be a good match.
I used the router with core box bit to cut appropriate size circles in two pieces of board the same diameter as the original surround roll, 80mm peak-to-peak. I used a homemade router circle cutter.
I then lay the foam sheet on top of one of the cut boards, place the tube ontop of that, then place the other board ontop of that, creating a sandwich that presses the tube into the foam and forces it down into the channel I cut. You can see I simply cut the tube the correct length to fit the circumference then taped the ends together- nothing very exact.I screw the boards together with a common wood screw, directly in the center in order to hold the sandwich together tight.
I then take the sandwiched boards and place it into an oven; I used my little toaster oven which has a "warm" setting. Don't know what temperature that is, but probably around 150degrees F. Leave it in for one hour, then turn the oven off and let it sit for a few hours so the sandwich (with the pressed foam inside) will cool slowly. After it cools I unscrew the sandwich and voila, the foam surround roll has been shaped!
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After that it is just a matter of cutting the surround to the proper size (a delicate job). In my case, the foam surround is glued from under the cone. I made a mess with the glue, but it doesn't seem to matter (except cosmetically). I did have one spot of trouble with gluing the new surrounds- I used the DATS to run a buzz test and one of the drivers needed help. I found that I could actually stretch the surround with my fingers until the buzz test passed. Of course I also needed to re-glue the driver gaskets back on. For glue I used Elmers Craft Bond Tacky Glue, $2 at Hobby Lobby- there's probably better glues to use (Aleene's).
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I had two of these speakers, which are in an MTM arrangement. I made surrounds for the two drivers in one of the speakers, and left the originals in the other alone in order to compare them. I have a DATS V2 to get parameters, and I use REW to get the FR. I was interested to see how the new foam would change both the paremeters and the sound.
Here are the main parameters:
Original .......Repaired with new homemade foam surrounds
Fs 44Hz ........Fs 128
Qts .5 ...........Qts 1.17
Qes .65 .........Qes 1.74
Qms 2.4 ........Qms 3.6
Vas 0.18 cu ft Vas 0.04 cu ft
So, that seems pretty different, but again I am new to this. I didn't know how the difference in the mass of the new foam would change things.
See the attached plots from REW. Again, since I'm new at this I may have setup the plots wrong, but I did them both exactly the same way so I believe that they are valid as a comparison (not for absolute values).
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At some point I will be working on designing new and better xovers for these.
The repaired drivers don't really sound any different to my ears than the originals. The plot shows they should produce a bit more output (SPL), but that's difficult for me to distinguish.
