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-   -   Concrete Horns (http://www.diyaudio.com/forums/multi-way/129734-concrete-horns.html)

MiWi 12th September 2008 04:47 PM

Moderator note: this thread split off from here http://www.diyaudio.com/forums/showt...hreadid=129625 :cop:

Hello Jean-Michel.


Quote:

The only questionable thing I can see is that you just applied some strain ratio to the initial circular section perpendicular to the axis in order to make it elliptical.
Thanks for the hint. (Btw, Im a big admirer of your work and follow it with big interest and pleasure)
I always considered this flaw from the beginning and decided to find out what it will finally will sound.
The horns were designed on CAD and fem or bem are available to me (structural engeneer), so I played with some versions that took an eye on that.
But: I wanted to give a big merit to the appearance for my eye.
And this way it just looked better to me (for my eyes this just looked "cleaner" or something like that).

I admit: With my constructions sometimes function has to follow form.

:smash:

But I guess though this, it measures quite good:
(The dip around 6kHz is due to the not optimal crossover of the bms 4590 coax. With a radian 2 the horn measures flat)

http://www.miwis-bastelbu.de/Galerie...lsteine140.jpg

Hi denewma04.

Before casting it in stone, there were prototypes made of fibreglass, papercrete and polymeric concrete.
Finally I made the flowstone-versions and they sound superior to the other versions. (though they all have the same measured response)

Last step in the row was to make a complete prototype-horn from fibreglass and plaster and then take a mold from this, which sonsist of 3 seperate parts.
http://www.miwis-bastelbu.de/Galerie...Wi-Horn009.jpg
http://www.miwis-bastelbu.de/Galerie...Wi-Horn012.jpg

The procedure is the same as with normal cement.
But the blend is very sensitive, were still experimenting.
Ingrdients (percentage still vary):
white cement (high resistance, pressure about 100N/mm, tension without reinforcement 15N/mm), sand, water, deaerate-additive, flow-additive, glassfibres, pigment if you wish to.
(The base of my TT is of this black-pigmented concrete: click for a pic )

Its not behaving like common concrete, less resonating, smooth (if the mold is), very rigid.
It can be polished like natural stone, but its as difficult as with the natural stone, cause these concretes get very firm.
I use very expensive diamond-tools to grind the flange.
The horns are casted in a bassins, which is flooded with water some hours after the fresh concrete is poured in. The horn stays in the water for 30 days.

To get it out of the mold we need pressure of 10 bar ( appr. 150 psi).
After some time the horns come out of the mold with a silky surface like this (or sometimes it breaks up in parts :xeye: )
http://www.miwis-bastelbu.de/Galerie...Wi-Horn015.jpg
Everytime I manage to get the horn out of the mold in one piece and without harm, I owe myself a good islay-malt and light up a cigarette. :D

Cheers, Michael

SunRa 12th September 2008 05:53 PM

Hello Michael,

Could you elaborate a bit why it is needed to submerse the mold under water? This discouraged me a bit..

It would be great if you could put together a tutorial on making these horns. There are just to few articles on manufacturing horns. Angelo did a great job in this regard.

I mean what do you actually use in making the prototype horn? You mention plaster but what is that exactly and what is the process? Do you also use fiberglass for the prototype?

I hope these are not too many questions. I just find your work pretty amazing.

dnewma04 12th September 2008 06:21 PM

Michael, thanks for the info. I am also in the planning stages of a DIY turntable and have been giving a great deal of thought to making the base out of concrete as you have done. It looks beautiful. More research on that will be required. I'd love to copy your horns, but it's a bit out of my league, currently.

Great work on your website.

MiWi 12th September 2008 09:18 PM

Hello.
Yes, the prototypes are made of fibreglass and plaster, there are several products in the boat-building, model-making and car-tuning-shops.

The mold has to be submersed under water, because concrete is hydraulically hardening and it is building crystalline CSH-phases, which means it is binding the water chemically. To much water will not be bond, to less water and you wont get all c and s to an h so to say. (its difficult to me to explain this in my bad english, sorry)
This very special concrete must be mixed with only a wter/cement value of appr. 0,15 which is very low. This guaranties very less microholes in the structure lateron, because there is very less water which is not chemically bound and therefor cannot evaporate an leave tiny holes. The concrete has to me mixed nearly one hour, before it gets creamy. The first 20 minutes you think "this wont ever be a fluid" because its like dust first. This is crazy high-tech-cement.
Putting the mold under water gives you a better surface, a more stable replication of colour (depends on temperature and microholes in the surface) a stronger concrete and it prevents a big amount of shrinkage (dehydration-shrinkage as well as the further shrinkage process). Another effect is to keep the temperature of the hardening concrete low, which is also good.
Less shrinkage means less microcracks etc.
If you dont put it under water, it is nearly impossible, to get it out of the mold. There is still one molding hanging from the ceiling with on horn in it, we couldnt even get it of with a jackhammer.

But all this doesnt work with usual concrete. Usual concrete also has less good sound signature, it rings way more than this one.
This concret is about 7 times more expensive, has highest strength, is self-compressing and you will have to use additives like deaerate-additive, flow-additive, etc. The blend is very delicate. The additives have to be weight out by gram.

You can search for furniture made of this concret etc. on google.
See this:
flowstone and this broschure

Cheers, Michael


BTW: The speakers shown above are from a friend with my horns on top.
This are mine:
http://www.miwi-design.de/Lautsprech...kelsteine0.jpg
http://www.miwi-design.de/Lautsprecher/slides/qs_22.jpg
and another one from a friend who made the concrte horn-project together with me:
http://www.miwi-design.de/Lautsprech...tugsdruck1.jpg

His speakers as well as mine are supported each by two tapped horn subs, which are not shown (partly build in the walls or kept under curtains etc.)

tvi 12th September 2008 09:18 PM

Quote:

Hello Michael, Could you elaborate a bit why it is needed to submerse the mold under water? This discouraged me a bit..
I would think it because the curing of concrete is exothermic?

thermal cracking


Regards
James

MiWi 12th September 2008 09:24 PM

Hi James.
Quote:

I would think it because the curing of concrete is exothermic?
Right.
300l of water gaurantees a nearly constant, not to high temperature for the hardening process (weather its winter or summer / warm or cold in the garage)

Michael

SunRa 12th September 2008 09:30 PM

This looks so alien stuff...


Thank you Michael for your informations. I'll start reading and maybe when I'll begin understanding more about these materials I'll start a thread or something :)

MiWi 12th September 2008 09:40 PM

Quote:

This looks so alien stuff...
:) And this concrete behaves like alien stuff.
You know this fluid Terminator from T2 ?
When you take the concrete after 1h of mixing, you still can make dusty breadcrumbs when grinding it in your hands (because of so less water in it). It feels very strong an rigid, you cant squeeze it through your fingers.
The breadcrumbs fall on the floor and after one minute they start to to converge, one minute later they contract to one flat spread mass, like this sh.. is living.
It needs time to flow and to self-compress.
The first ten minutes when the horns are in the mold, a lot of bubbles ascent and the concrete itselfs gets rid off all the air in it.
Im a strutural engeneer and have seen some concrete, but never one strange like this.

Michael

soongsc 13th September 2008 12:41 AM

This look so nice I just have to see what goes on.

SunRa 13th September 2008 11:47 AM

@Michael,

at the risk of becoming annoying, do you think it would be ok to start a new thread and discuss a bit about the materials you used?

Probably many of us are not that familiarised with different fibeglass/plaster materials and techniques. Not to mention alien concrete :) . I mean, the brands you are using and so on.

I guess that starting a new thread or continuing the one about your speakers posted several months ago would be better, in order to preserve Angelo's subject.

Oh, and by the way, you have mentioned listening to the radian driver in the same horn. How do you compare it with the coaxial one?

Thank you!


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