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Old 14th December 2006, 07:55 PM   #1
Dr_EM is offline Dr_EM  United Kingdom
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Question Concrete speaker

Would it be any good to make a speaker out of concrete? Since it can be poured into a moulding, you could make complicated horn designs more easily. Its certainly very dense which is a good property I believe? Of course problems would occure trying to make anything other than small bookshelf type speakers due to weight, but for those smaller speakers it might be worth a try.

My idea for moulding one isn't fully formed, but could go a little like this. Mould the open horn space out of plaster of paris (sp?) using hands and hand tools (probably a better way). Once set, fix to a wooden framework. Pour in concrete around it. Hopefully the plaster comes away from the set concrete easily (vaseline the plaster?), leaving the horn passage/mouth etc. Turn over to pour the other side panel. The front and back would just be wood, fixed with wall plugs and screws.

I see potential difficulty in removing the plaster mould since if it isn't symmetrical it will become wedged. It is possible to do 3D type moulds I would think by casting half at a time. Any thoughts?
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Old 14th December 2006, 08:27 PM   #2
AMV8 is offline AMV8  United Kingdom
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Hi

I have previously built concrete speakers. My current speakers use a midrange enclosure made of concrete/corian.

My experience has shown that the rigidity that concrete offers is only necessary for the mid range unit. I found that a rigid wood cabinet sounded as good as a concrete cabinet for the base enclosure. I have read similar comments from others.

My last speakers were made from Corian as I found it easier to work and did not need a finish added as the Corian could be polished.

I hope this helps

on
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Old 14th December 2006, 10:32 PM   #3
Dr_EM is offline Dr_EM  United Kingdom
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Thanks. I actually thought it might be easier to use was my main reasoning with it, me not being experienced with wood shaping or having the tools.

It would probably be a full range, but I'm not sure as it'll need to be big to get a good bass and then it'll likely be too heavy.

Is your midrange a spherical shape, like on B&W 800?

I assume concrete is quite cheap too? Not sure about corian, looks better but harder to get and more expensive?
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Old 15th December 2006, 12:07 PM   #4
Dr_EM is offline Dr_EM  United Kingdom
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Sorry to double post, I don't see edit feature. Anyhow, i'm lloking at these:

http://www.spnet.ne.jp/%7Ehasehiro/p...kroadhorn.html

(babelfish will give some degree of translation). They are complicated horns, but could be moulded from concrete relatively easily (possibly...).

Some things I find interesting are that the smaller models (like I want to attempt) use the Fostex FE87E; a driver that I thought had too high a Qts for rear horn loading . I thought between 0.2-0.3 were the only suitable values? The FE87E is 0.92 . Presumably it works though, they arn't cheap on that site (115 for the smallest one).

Also it suggests similar replacement drivers. Does this mean it is a fairly non-critical design since it will readily accept drivers with differing parameters? I'm hoping so since I may be able to do one if thats the case . I'm not sure about dimensions. It appears, and the translated text suggests, that the initial folded section is straight and does not expand at all, but that doesn't seem right. Is it an exponential horn of sorts?
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Old 15th December 2006, 12:14 PM   #5
soongsc is offline soongsc  Taiwan
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Quote:
Originally posted by AMV8
Hi

I have previously built concrete speakers. My current speakers use a midrange enclosure made of concrete/corian.

My experience has shown that the rigidity that concrete offers is only necessary for the mid range unit. I found that a rigid wood cabinet sounded as good as a concrete cabinet for the base enclosure. I have read similar comments from others.

My last speakers were made from Corian as I found it easier to work and did not need a finish added as the Corian could be polished.

I hope this helps

on
What would the mold be made of if you work with Corian?
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Old 15th December 2006, 01:41 PM   #6
Geoff H is offline Geoff H  Australia
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A friend of mine was building concrete enclosures in the 70's. They were full sized floor standing units, and sold commercially.

The problems he had to overcome:

Weight: Mix in some polystyrene beads. The mix is still inert.

Failure rate: The mould must be placed on a vibrator to allow the mix to reach into the corners.

Finish: Gluing veneer to the mix, the glue must bond well, without dissolving the polystyrene. I forget what was used. He did consult with BASF and 3 M.

Is it worth it. You bet. Soncally better. Cheaper to produce, once the buggs are sorted out.

Have fun,

Geoff
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Old 15th December 2006, 05:04 PM   #7
Dr_EM is offline Dr_EM  United Kingdom
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Cool, thanks for that . I expect i'll be making small enough speakers to not require polystyrene beads, but thats a neat trick. Well, I might need some, depends on the design I end up with.

Didn't even consider about the vibrating mould. To implement it I can use some motors I have around fixed with off centre weights, should do the trick. Presumably just sit the mould with the wet concrete on that for a few minutes to get it into all the corners? Possibly more will need to be added whilst this happens as the level would be expected to drop.

Hadn't thought as far ahead as finish, i'm mostly persuing a project for my own interest, but i'll be sure to be careful if I do veneer it.

For designs, i'm looking at the fostex reccomended enclosures. In particular the FE87E and FE103E (they are cheaper drivers ). They are clearly designed for simple construction. My plan would be to hopefully improve it when making them from concrete by smoothing everything. I mean rounding the corners and making continuous changes (thier designs suddenly change from say 25mm diameter to 35mm, I could have it changing continuously which I believe is better ).

If it is made to change continuously, how do I calculate it? It isn't linear is it, it's exponential? IE, if it changed from 25mm to 35mm over a 20cm course, it wouldn't ideally be 30mm at 10cm right?

I've also had more thinking on the moulding. I hope to be able to make it from thick cardboard stacked up the edged with thinner bendable card. It would certainly be easy to work with. A really nice plan would be to make a straight horn out of a material which can then be folded and drawn around as a template since that would make things easier/more ideal. Don't know of any such material unfortunately so it'll be compasses and pencils
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Old 15th December 2006, 06:04 PM   #8
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This may be helpful.
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Old 15th December 2006, 06:26 PM   #9
Nordic is offline Nordic  South Africa
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I once read an article where a guy described a few versions he went trough when building his concrete boxes. He settle for concrete mixed with foam strips cut from those long sponge floaty things you get in swimmingpools...

I thought about trying to make a cube, and put a ball in the centre, then fill the cube, with neccesary precaution, like wide tube stuck to top of ball to keep speaker opening..., when the conrete sets you just need to pop a hole in the ball and remove it....
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Old 9th February 2007, 08:57 PM   #10
Dr_EM is offline Dr_EM  United Kingdom
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Hi again. I'm thinking about this more now but need some more guidance. Basically I want to make the Fostex FE87E design rear loaded horn but as a real smooth horn rather than a more crude, easily constructed, type.

I'm wondering firstly if that will have much impact and also how is best to approach it. Would I simply find the lengths of all the passages and curve each one or would it be better to say the throat starts at X mm wide and ends at X size mouth, with a length of X, then apply a function to define the best curvature.

http://www.fostexinternational.com/d...e/87e_encl.pdf

Think I know how to go about the casting now, I've thought that all through pretty much. I will probably make an enclosure/cast first before buying drivers anyhow just to see if it does work properly.
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