Reading how you plan to go about making this horn, I offer the following (this first one you probably aready know):

Make the outside of the mold in quarters or thirds and hold it together using fabric straps (not bolted flanges)... If you make it in halves the mold will split due to expansion of the casting material when the chemical reaction causes it to heat up... The straps will flex just enough to allow expansion.

Also, make the rear chamber plug a separate part from the main "volcano" form... This will make the whole thing a lot easier to pop out of the mold, as you will not be forced to unmold a multi-hundred-pound part that will bind due to the double draft conditions (tested!!!!). It will also allow you to fit different diameter rear chamber plugs later on.

Also, when using a template to form the plaster, the key is to load the plaster on and turn or cut it while it is still very very  liquid... Just keep splashing it on and turning (really a two-man job). Obviously, with a horn this big you will want to rotate the template, and leave the assembly stationary... The challenge will be to make sure the rotation template is RIGIDLY mounted at both top and bottom, and is held in a RIGID frame.

I use polyester resin to seal the plaster surface, then floor wax (the real stuff... the kind intended for hardwood floors) as a mold release... it is way cheaper than buying "Mold Release Wax"... I used to complete the process with a coat of PVA (poly-vinyl-acetate, a commercial mold release agent), but this is just a waste of money... 2 or 3 heavy coats of floor wax does the trick.

Get on a high-carbohydrate diet one month before starting work!

I look forward to seeing the images!

jd*

jd wrote :

"...Obviously, with a horn this big you will want to rotate the template, and leave the assembly stationary... The challenge will be to make sure the rotation template is RIGIDLY mounted at both top and bottom, and is held in a RIGID frame..."

On second thought, because the horn must sit on something while you turn it, I can't see a simple way to rigidly locate the bottom of the template while still allowing it to freely rotate about the horns central axis... Probably better to bolt the template to a corner of your house and get three pre-carbo-loaded friends to help rotate the horn.

Alternatively you could do what I did... I rotated the master for my upper bass horn in halves, with the horn laying horizontally... Because the template did not have to rotate more than 180°, this allowed simple fixation of the template at both ends, to the central axis (the other reason for doing it in halves is getting it through the door).

jd*

ulf wrote:

Let me tell you a bit about how an why we designed the midbass horns the way we did. A few years ago we discovered the tractrix horn for MF use. It was really the most important improvement so far. No more honking exponential horn sound. The improvement was so clear that A/B testing was not needed.  My friend made a pair oval 350Hz horns of concrete and I made a pair of round 240Hz horns of MDF. We compared these horns and were not able to detect any audible problems with the oval horn. So we figured that the Tractrix curve may not be so important. Both horns have the flat termination of the tractrix and this seems to be the most important issue that differs from exponential horns. After a while he wanted to upgrade his 90Hz Hypex midbass horns and we had long discussions about the design. We made some tests with the existing 90Hz where we tested some different drivers, phase plug or not and back cavity. We settled on a design that would be close to tractrix, use phase plug and back cavity and the selected driver was the TAD 1201H. The 1201 is really suitable for midbass with its low Q, superlow mms and high Fs and BL. In the listening test it sounded cleaner than the other drivers we tested. Although I'm not a fan of composite diaphragms, I really liked the 1201 in this application. Given the size of the horn we wanted the mouth to rest on the floor. A true tractrix is designed to work in free space and a horn of this size placed in free air would really take a lot of space. So we used a modified tractrix that would better mate to the floor. On 3 of the lips the tractrix is clearly visible, the lip that rests on the floor is a bit flatter. After many simulations in Hornresp (thanks David) we had a design that we were satisfied with. Still we were not quite shure about how we should design the phase plug so we decided to not include it in the concrete horn. The plug and the first (about  15 cm) segment is bolted on the concrete horn. Making the mould was a labour intense task!! Pouring the concrete and removing the moulds was nice and really exiting. You just have to be careful! The horn weighs about 300kg and the mould maybe 100 so watch your toes! The preliminary results of the horn without the phase plug was rather disappointing, but once the plug was in place we were quite happy with the result. Compared to the old 90Hz hypex we got a cleaner, more controlled sound. Over all we only could find positive changes. The most amazing thing though is the soundstage. It really sounds large. A surprising amount of stereo information were revealed that we didn't expect. It mates really well with the MF horn and it is virtually impossible to hear the crossover. A measured response showed that we had some more output in the upper region than we wanted. Up at 800 it is about 3dB too much and this remains a problem. So there is still room for improvement. Romy, yes the horn will transfer more HF POWER. But the phase plug helps distribute the HF so the level at the sweet spot may actually be lower (I think). A comment about the directivity pattern that I included: Although it is not measured you easily can hear it when moving around your head close to the horn moth. With this phase plug the response is more uniform compared to the same horn without the plug. Without the plug the HF get more attenuated when you move from the middle of the mouth. An image of the plug. You can see how the HF from the left side of the diaphragm will beam to the right side of the horn. The plug follows the tangent of the horn side. Throughout this process we have had many "nerd meetings" when a group of people of similar interests have gathered and listened and compared different solutions. Even though not all have been Horn Fundamentalists we were able to reach consensus on all the issues. All of us share an interest for music and technology. Frequent visits to live performances are most important. Any visit to the local High-End dealers are only beneficial for getting a good laugh and it makes you wonder how people can spend so much money on crappy (but usually good looking) equipment. Best Regards: Ulf