TQWT Concrete cabinet attempt, building thread. - diyAudio
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Old 15th June 2009, 11:25 AM   #1
h@kan is offline h@kan  Sweden
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Default TQWT Concrete cabinet attempt, building thread.

I have always wanted to build a pair of speakers in concrete but I have never find a way to do this in just one casting. The problem has been to get the form out from the finished cabinet. I talked about this with my colleague at work (im a civil engineer) and he suggested to build the form out of polystyrene and then either melting it with heat or chemically with thinner (Toluen) after the concrete has solidified. After testing the methods on a piece of polystyrene I decided to go for the chemical approach in my project.

Since I'm into transmission lines (at the moment) I decided to build a tapered quarter wave tube based on Martin J King Mathcad sheets, that I downloaded back in 2006 or so, but not yet have had the opportunity to try out. I'm sure the new updated (but unavailable to me) sheets are better but since this is more of an experiment in using concrete in the design than building the most optimized TL out there I'm happy that I still had the sheets on my harddrive.

The driver of choice is the Mark Audio CHR-70 that I have heard good things about.

After some hours altering the parameters in Mathcad I was quite satisfied with the teoretical result (as far as I knew with zero experience in designing TL's). So then I jumped on the building. The fun stuff in my opinion

First of all you need some special tools before the real work can start:

1. Someting to cut the polystyrene with: A simple "hot wire" saw.
2. Something to rotate the 60 kg heavy speaker with under the thinner melting process: A monstrous 3D-Rotator

Enough speaking, here are the result so far:

Hotwire saw

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The tube cut out from polystyrene sheet

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The edges are rounded a bit to give the concrete structure more strength

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The raw tube form in the plywood form

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Some details "shaped" in the polystyrene. Think of the polystyrene as a negative of what will become the cavity inside the concrete. The triangular shape will be a multi purpose cavity serving as terminal and holmholtz resonator or just as terminal depending on the results with the resonator.

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The concrete is vibrated just enough to fill out the form.

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Old 15th June 2009, 11:27 AM   #2
h@kan is offline h@kan  Sweden
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The speaker is mounted in the monstrous 3D-Rotator
I have no pics of the actual thinner melting process while it was quite messy and I did'nt wanna melt my camera aswell.

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At this time all the polystyrene was gone, I tested to fill the tube with water to se if all polystyrene was gone and the water volume matched the theoretical volume.
The tube is around 25 liters and to melt the polystyrene I used about 2.5 liters of thinner. Don't try this indoors, the fumes was quite heavy even when working outside a windy day.
I rounded the edges and rubbed it with cement to get a raw concrete look.

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This is where I am so far and I have started to play around with stuffing the pipe and different setups with the helmholtz resonator.
I have done some measurements but I dont know how relevant they are since my equipment is very "diy" (and my knowledge aswell), Panasonic WM-61A mic anf a cheap diy mic preamp.
I will post the graphs later anyway.
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Old 15th June 2009, 12:11 PM   #3
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Fascinating!

I'm really interested in your hot wire saw. Did you make it yourself? What kind of wire is it? What voltage?
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Old 15th June 2009, 02:07 PM   #4
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For those who are attempting this in the future, straight delimonine will dissolve polystyrene. It is a little less volatile than other solvents.

delimonine MSDS http://www.supercoproducts.com/msds/MSDS-FLOAT.PDF
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Old 15th June 2009, 02:30 PM   #5
bjorno is offline bjorno  Sweden
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Hi h@kan,

Nice project! Your hot wire saw set-up is genius, I will copy using the closeness of a window to avoid the smell of hot polystyrene.
Do you know that light weight ' EPS cement', http://www.epscement.com/ , have 85% less density than ordinary concrete mix, i.e. the weight could be reduced from 60 kg to about 9 kg without sacrificing good mold stability.

b
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Old 15th June 2009, 02:46 PM   #6
udailey is online now udailey  United States
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Way to DIY!!
Very nice idea and congratulations on your idea working! Will follow the thread for your listening impressions.
Uriah
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Old 15th June 2009, 02:48 PM   #7
qingcai is offline qingcai  United States
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how is the sound?
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Old 15th June 2009, 05:49 PM   #8
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Well done... not a whole lot of taper in the TQWT?

dave
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Old 15th June 2009, 06:01 PM   #9
yan6 is offline yan6  Canada
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I'm not sure on the scale, but I am wondering if the polystyrene could be replaced by plumbing, ie: 4 or 6" sewer pipe. This way you wouldnt have to melt it out and it would be smooth inside. Of course you are then limited to the shape of a circle?

Are these for outdoor use? What kind of driver will you use in them?

Great project! this sure sparked my interest if for outdoor use.

Keep us posted
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Old 15th June 2009, 10:22 PM   #10
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The creativity of people on this site never ceases to amaze. Excellent idea brilliantly executed. Looking forward to seeing more pictures and reading how it sounds.

Martin
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