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Old 23rd February 2012, 06:23 PM   #1
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Default How to Make Concrete Horns and Enclosures

I've been studying horns, waveguides, and loudspeakers for over ten years. I've obsessed over horn curves, and drivers, and crossovers, etc.

Click the image to open in full size.
A few years ago I bought myself a set of Gedlee Summas, and realized there was an aspect of loudspeaker design that I'd overlooked.

The shape and the rigidity of the enclosure is INCREDIBLY critical. My Summas are depressingly heavy, but they're also solid as a rock. They're made from a composite of MDF and carbon fiber. (Please excuse the mess in the pic above, I had to build my subwoofer inside the house, as it wouldn't fit through the front door of the house.)

For most of the last decade I've built my horns out of fiberglass or carbon fiber, but I stumbled across an interesting alternative today that some of you might be interested in:

Concrete Casting: How to Make a Concrete Sink Mold - YouTube

The video above shows how to make a concrete sink - but the instructions could be used to make a concrete loudspeaker, or a concrete horn.

This fabrication method has some excellent advantages over fiberglass or carbon fiber:
  • it's dirt cheap. The amount of carbon fiber required for a loudspeaker enclosure is hundreds of dollars; in concrete it's about $20
  • Making a horn out of carbon fiber is labor intensive. I've easily wasted DAYS making carbon fiber horns, because you have to 'build it up' layer by layer by layer. With concrete, you pour it once, and you're done.
  • fiberglass is cheaper than carbon fiber, but fiberglas is even MORE labor intensive, because it requires more layers. And depending on the resin, fiberglass shrinks! (ugh)
  • unfinished carbon fiber is degraded by ultraviolet light
  • Probably the greatest advantage of concrete is it's mass. I've noticed that fiberglass horns resonate A LOT. Won't be a problem with concrete.

If I were going to make a concrete enclosure or horn, based on the video above, I'd make the mold negative out of MDF. In all other regards, the information in the video could be followed to the letter.

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Old 23rd February 2012, 06:53 PM   #2
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Note : I am no expert on concrete, so someone correct me if any of this info is correct.

With that disclaimer out of the way, here's a quick list of what you need to do this, based on a bit of research.

#1 - You'll need some MDF to make the mold negative (whether it's a horn or a loudspeaker.) Should be able to get that at any local lumber store.
#2 - According to the video, the mold rubber is Vytaflex 40. It's $105.43 and available here: VytaFlex® Series Urethane Rubber Product Information | Smooth-On
#3 - You'll need mold release, $23.10, available here:One Step™ Release Agent And Sealer Product Information | Smooth-On
#4 - I honestly can't figure out what type of concrete is required. Can you use plain ol' concrete from Home Depot? If so, that's a whopping $3.65(!) Building Materials - Concrete, Cement & Masonry - Mortar, Cement & Concrete Mix at The Home Depot
Click the image to open in full size.
#5 - Gotta make it pretty right? That will be $19.95 for a concrete acid stain... DIY Concrete Acid Stain | Direct Colors
#6 - and $20.65 for the sealer: DirectColors.com - Concrete Sealers for Any Project | Direct Colors

If I didn't make any mistakes in my pricing, it looks like a concrete waveguide is an order of magnitude cheaper than a carbon fiber waveguide. Heck, it looks like it's even cheaper than MDF. Wow! Considering how cheap concrete is, you could probably knock out a pair for under $20. (Of course you'll end up investing a few hundred in the mold.)

Click the image to open in full size.
Who's going to be the first person to knock out some Magico-style horns?


#5 -
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Old 23rd February 2012, 06:58 PM   #3
juz400 is offline juz400  United Kingdom
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Excellent Idea

I have a Faraday Sound concrete cabinet passive sub, 8" driver with 3" dia port 5.5" long and Cabinet is 11"x11"x18"
It looks to be cast in two pieces, front baffle is 1.5" attached to what looks like a 1 part cabinet.

It is impresive for its size, good for both movies and music

I think the guy who made them in the UK had been making concrete cabinet speakers for many years but has retired now.
He does a little info book on ebay every now and then just search the name above

I`d love to build a 10" version but I have no where to do it.
I assume there must be a metal reinforcment inside the concrete, would that be difficult to support inside a horn?

The upside is that if you create a wonderful horn, you could share that with many and everyones would be the same!
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Old 23rd February 2012, 06:59 PM   #4
sreten is offline sreten  United Kingdom
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Hi,

Concrete is :

Cement + sand + aggregate.

Lots of different flavours.

rgds, sreten.
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Old 23rd February 2012, 09:02 PM   #5
C37 is offline C37  Belgium
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Quote:
Originally Posted by sreten View Post
Hi,

Lots of different flavours.
You might want to investigate a form a concrete called "Granito' in French, a concrete containing a mix of tiny marble chips. This is what was used in platine Verdier's base ( bottom picture here JC Verdier products)

It was choosen by Verdier because 1) it could be molded, 2) The chips scatter standing waves inside the material as they are not as homogenous as "normal concrete"

Once polished, it can look stunning. Lots of older houses have this in Belgium, as it was less expensive than marble. I have always thought one day i would build speakers out of this :-)
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Old 23rd February 2012, 09:09 PM   #6
C37 is offline C37  Belgium
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PS: This thread should really be in the "Construction Tips" section...
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Old 23rd February 2012, 09:18 PM   #7
Irakli is offline Irakli  United States
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I have made concrete speakers back in 90's. I used premixed material from Home Depot. Which one I do not remember, but it was labeled "for fine work" or something like that. Just added right amount of water and poured.
The molds for the panels I made from ordinary plywood which I covered with vaseline before I poured concrete.
Another point to consider is using steel enforcement. I used steel wire mesh which was also bought in Home Depot.
In my opinion, concrete is very nice material to work with (messy though).
So, go for it
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Old 23rd February 2012, 09:22 PM   #8
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the best for horns...is.... PAPER and WHITE GLUE!
It's a very good combination. I have made a trial with Inlow's method ...and i was impressed.
Maybe i just like the resonances that paper produces.

I don't know why, but i have a feeling that it is better to have a horn with soft walls and high mass.
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Old 23rd February 2012, 09:27 PM   #9
Irakli is offline Irakli  United States
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Quote:
Originally Posted by rvrazvan View Post
I don't know why, but i have a feeling that it is better to have a horn with soft walls and high mass.
Isn't good horn wall reflectivity essencial for good sound reproduction? How than soft walls help?
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Old 24th February 2012, 04:50 AM   #10
ra7 is offline ra7  United States
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Inlow's horns are beyond brilliant... the smoothest, most realistic sound you will ever hear. I have a pair
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