Two-way coaxial loudspeakers with molded waveguides - diyAudio
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Old 23rd December 2013, 07:43 PM   #1
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Default Two-way coaxial loudspeakers with molded waveguides

As I promised in the new member introductions, here’s my project introduced.

Making small coaxials is perhaps not the most exotic project but I figured out a useful way to make waveguides from epoxy/fiberglass. Those who have made molds know that it can be quite tedious.

I started this project because I need new front speakers in my music room/home theatre. The room has a bit difficult acoustics and I don’t want to start complete renovation to improve it. Thus I took the route of making speakers with controlled directivity. The drivers will be 8” coaxials (18Sound 8CX400F). I did some modeling using wavefront simulator of Hornresp and came to conclusion that a short conical horn should work.

So here is how I made the mold:

First I made a plywood frame with an opening which has the exact shape of the mouth of the waveguide. I also cut a disc which has the diameter of the throat.

kehikko1.jpg

Then I stretched thick construction plastic over the frame using lots of staples and gaffer tape.

muovi2.jpg

Careful heating with a heat gun took all the wrinkles away.

muovi_kiristetty.jpg

The shape of the waveguide starts to form...

muotin_muoto1.jpg

Some measurements in order to ensure that the disc is clamped to the correct position...

muotin_muoto2.jpg

First I brushed a layer of epoxy with some colloidal silica mixed into it and then I laminated two layers of 300 gram glass cloth over the plastic. Not shown in this picture, but I reinforced the edges by two additional layers of 300 gram glass cloth. After this, I let the epoxy to set for two hours.

kuidut_pinnassa2.jpg

Then I applied a 1 - 2 cm thick layer of mixture of laminating epoxy and sandblasting sand. This will make the mold very strong.

epoksihiekka.jpg

The mold is reinforced by a MDF sheet...

taustalevy.jpg

…and here’s the mold. The shape is perfect and the surface is smooth but unfortunately there had been some air bubbles between the plastic and the laminate. I want the surface to be perfect and I will fix the bubbles and paint the mold with automotive filler/primer. That has to wait until the Christmas is over.

muotti_irrotettu.jpg

Merry Christmas!
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Old 23rd December 2013, 08:01 PM   #2
tinitus is offline tinitus  Europe
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Originally Posted by Erkki Rasanen View Post
I want the surface to be perfect and I will fix the bubbles and paint the mold with automotive filler/primer.
niiice

hmm, if its a mold, you would have to do careful wet sanding and lots of polishing in any case anyway

but I suppose you already know this, and how to do it

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Old 24th December 2013, 10:11 AM   #3
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Originally Posted by tinitus View Post
niiice

hmm, if its a mold, you would have to do careful wet sanding and lots of polishing in any case anyway

but I suppose you already know this, and how to do it

Yes, normally I would apply 2-k primer and thick layer of top coat, wet on wet... Then wet sanding and polishing. Tedious and messy.

In this case I want matt surface and I just apply primer and do wet sanding only. This gives nice satin-like surface and much less work is needed.

Last edited by Erkki Rasanen; 24th December 2013 at 10:29 AM.
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Old 24th December 2013, 12:08 PM   #4
tinitus is offline tinitus  Europe
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well, its obvious you know how it works

will be interesting to follow
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Old 24th December 2013, 02:42 PM   #5
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Very nice work Erkki.
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Old 25th December 2013, 12:56 AM   #6
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Yes indeed I'll be watching this one since I have a Fostex co-ax to do something with besides the crappy box it came in back in the 80s.
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Old 29th December 2013, 07:10 AM   #7
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The project continues…

The surface of the mold was quite good and needed only two layers of primer paint. Then I wet-sanded it using 800-grit sandpaper.

kuva 2.JPG

The surface must be waxed at least six times. Hard carnauba wax is the best for this purpose, liquid and silicon waxes must never be used. The wax must dry a couple of hours after each waxing.

kuva 1.JPG

Meanwhile I cut three rings of 4 mm plywood. These will support the drivers.

20131228_123615.jpg

After waxing, release agent is applied. Release agent is water soluble polymer that forms a very thin film on the surface of the mold. Extra-strong hairspray is very good for this purpose. First I sprayed a thin mist on the surface, let it dry for 15 minutes, and then I sprayed more generously.

20131228_132158.jpg

Everything must be ready and available when the laminating work begins. I cut 20 pieces of 80 gram and 300 gram cloth, size of them is about 10 x 15 cm. Pieces of sponge are used for laminating; first I spread epoxy resin on the surface, then I apply pieces of cloth by patting them gently on the surface with a sponge.

20131228_134753.jpg

Epoxy does not smell bad but it is very harmful. Personal protection is of utmost importance. I protect myself with cotton and nitrile gloves and wear a gas mask, and I have good ventilation in the room.

First layers of thin 80 gram cloth have been laminated. Thin cloth on the surface prevents formation of small pinholes.

20131228_142340.jpg

Several layers of thick 300 gram cloth…

20131228_145103.jpg

The driver support ring is aligned to correct position…

20131228_150541.jpg

…and pressed against the wet laminate by some weight. I will reinforce the support ring afterwards.

20131228_150930.jpg

And voilà, an almost perfect waveguide! It released easily from the mold, and the mold must be cleaned and waxed again before making a new one. I will make the rest of them during the next week.

20131229_094340.jpg
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Old 29th December 2013, 02:08 PM   #8
OllBoll is offline OllBoll  Sweden
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Looks beautiful!

I just wander, was there a specific reason you decided to go with a rectangular waveguide and not a round one?
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Old 29th December 2013, 03:18 PM   #9
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OllBoll; no specific reason, I just ended up to rectangular shape, although I was thinking of superelliptical shape also.

Last edited by Erkki Rasanen; 29th December 2013 at 03:21 PM.
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Old 29th December 2013, 04:00 PM   #10
tinitus is offline tinitus  Europe
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Quote:
Originally Posted by Erkki Rasanen View Post
... I was thinking of superelliptical shape also.
elliptic might be a really great match for your method
and shape the wood edge round, where the poly film is pulled down
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