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MJL21193 30th January 2010 03:44 PM

Casting Experiment: Elliptical Horn Flare
4 Attachment(s)
split off from :cop:


Originally Posted by MJL21193 (

Moving to a compression driver is something I've been meaning to try and this is a good opportunity.
The horn flare will not be a problem to construct - I won't need a CNC (there were and are people that can fabricate accurately without them) and I'll try to go through the process with a few pics to show what I'm doing. I'm a bit pressed for time right now with a few different things happening but I may find time on the weekend to get started on the flare..
I don't want to cross the HF driver too low - 2-2.5k is where I want to be.

I'll take a side track into the wild world of horn flare design and construction.
The concept:

Attachment 155969

Top view:
Attachment 155970

Side view:
Attachment 155971

My original idea was to cut this shape right into the front baffle, as I did here:
Attachment 155972

That is labour intensive and pretty much one-off, my hard work goes into the baffle and that's it. I wanted to play with mold building and casting so where to find a better place to start than with a horn flare :)
Basic idea is a elliptical flare, conical in shape with 90 degrees horizontal dispersion, 40 degrees vertical.

Magura 30th January 2010 03:56 PM

Make a positive mold, throw it in an oven at like 250C, with a piece of 3mm PET on top. Wait a few minutes, take it out of the oven, trim.

Presto! and you got a real good looking horn flare.

Magura :)

EDIT: I think it's about time, to direct your attention towards your we are 13 whom will laugh when it won't work :rofl:

MJL21193 30th January 2010 04:06 PM

3 Attachment(s)

Originally Posted by PKNJ (
I have to say I'm impressed with the thought put into the cabinet design. And you even thought out the construction in detail as well. I convinced it will work the way you think it will. Most of the naysayers seem to have fallen away. I voted "It'll Work" early.

Thanks :)

More of the design concept:

Attachment 155973

A section view through the centre axis vertical and horizontal showing the details of the throat and mouth.
To construct a mold for this shape is tricky (obviously) but not outside the realm of possibility. Suggested was a CNC machine to cut the profile but I don't have access to one of those. I'm relatively old school in any case and can usually come up with a way of doing things

It starts with cardboard. From cardboard I lay out and cut an ellipse of the mouth. This will be where the sides of the flare meets the front baffle, without a roundover. I then make a cone, ordinary straight round cone from paper. Trial and error gets me to the exact dimensions after a few tries and I wind up with this:

Attachment 155974

That is the ellipse cut out from cardboard (Frosted Flakes cereal box side :up:) forcing the cone into the correct shape. The throat opening of the cone is 1".
Using a careful, steady hand I trace around the cone where it meets the cardboard, cut on the line on the paper to get this:

Attachment 155980

I then transfer that shape to cardboard.

MJL21193 30th January 2010 04:11 PM


Originally Posted by Magura (
Make a positive mold, throw it in an oven at like 250C, with a piece of 3mm PET on top. Wait a few minutes, take it out of the oven, trim.

That's not a bad idea! I'll try that I think. ;)


Originally Posted by Magura (
EDIT: I think it's about time, to direct your attention towards your we are 13 whom will laugh when it won't work :rofl:

I'll laugh too. :D

MJL21193 30th January 2010 04:27 PM

1 Attachment(s)
next step is to make the form. I use a scrap of OSB and screw a large dowel to the centre, making sure the it is perpendicular. The dowel I used (old broom handle) was a bit small so I had to add a couple layers of cardboard and masking tape to build it out to 1".
The cardboard elliptical cone is then slid in place with the dowel through the throat hole:

Attachment 155981

I line it up and trace around it where it meets the OSB. I use silicone on this line to glue the cone in place.

MJL21193 30th January 2010 04:40 PM

2 Attachment(s)
After the silicone cured, I sprayed on some polyurethane to seal the cardboard then I drilled a few hole in the back of the OSB to access the cavity. I used spray foam to fill the cavity and let that cure overnight.

Next came some filling and sanding:

Attachment 155983

More filling, sanding and several coats of polyurethane:

Attachment 155984

It is glassy smooth now and is getting several coats of wax as a release agent.

MJL21193 30th January 2010 07:34 PM

2 Attachment(s)
With the "plug" built, I built a form on it to make a negative of the shape:

Attachment 156004

While the plug is very near perfect, it can be improved upon and this gives me the opportunity. Making a negative from plaster of Paris will serve two functions: First, it will allow me to create a smoother and more perfectly shaped positive for future castings since I can correct any of the defects in the negative before casting a new positive.
Second, once the new positive is created, the negative can be cut in half and modified to form the outer cast for backside of the horn flare.

It took 5 litres of plaster to fill:

Attachment 156005

I'll let this set overnight and reveal my masterpiece tomorrow. :)

Dr_EM 30th January 2010 07:41 PM

Good looking work on the horn casting! Look forward to seeing how that comes out :)

Cal Weldon 30th January 2010 07:55 PM

Very nice indeed John.

SunRa 30th January 2010 08:12 PM

This is a great tutorial :) Keep it coming!

By the way, what did you use for making those rendered images / drawings of the waveguide?

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