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Multi-Way Conventional loudspeakers with crossovers

First attempt at making homemade surrounds
First attempt at making homemade surrounds
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Old 12th July 2018, 02:35 AM   #1
keithostertag is offline keithostertag
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Join Date: Apr 2012
Location: New Albany, IN 47150
Default First attempt at making homemade surrounds

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!

Click the image to open in full size.


Click the image to open in full size.

Click the image to open in full size.

Click the image to open in full size.

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).

Click the image to open in full size.

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).

Click the image to open in full size.

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.
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Old 12th July 2018, 02:40 AM   #2
Cal Weldon is offline Cal Weldon  Canada
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First attempt at making homemade surrounds
Nice job. Haven't seen anyone make their own surrounds. Never mind the mess, function over form.
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Old 12th July 2018, 02:55 AM   #3
afa is offline afa  Australia
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Well done Keith, real DIY, what a great idea, I might have to try that process to make some of those hard to get square surrounds. When I get foam surrounds from my suppliers, sometimes they have the original uncompressed foam pieces still in the centre, this foam is usually light weight, open celled and about 12mm thick, it is like the top layer of the foam on the top of a foam mattress, I think it is either polyurethane or something similar.
cheers, Arthur.
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Old 12th July 2018, 04:23 AM   #4
DaveG is offline DaveG  United States
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excellent - thank you for sharing the process.
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Old 12th July 2018, 05:26 PM   #5
keithostertag is offline keithostertag
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Join Date: Apr 2012
Location: New Albany, IN 47150
Thanks guys.

I'll add: I am not convinced that the channel needs to be cut with a core box bit- it may be that a regular straight bit of the right size will do... don't know if the shape of the roll is critical. Perhaps I will try that next time I have some woofers to refoam...
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Old 13th July 2018, 12:26 AM   #6
phivates is offline phivates  United States
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Core box bit seems like the right approach. It worked. I have a pair of those Koss btw.
Looks like they are stiffer than the originals but hey the results are decent.
I have a pair of Audax PR 17 HR 100 1AK7 midranges, the very high output 7" cones which have flat foam surrounds needing replacement, and I was going to ask if anyone has tried felt for repair, but you've directed me in an easier direction. I don't care if the Fs goes up 50 hz, which would bug me if I was doing woofers. Nice work.
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