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ulf wrote:

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.

For whatever reasons tractrix rules only MF horns. For upperbass horns, I feel, tractrix it is not necessarily the only way to go, although my personal upperbass horn is religiously tractrix – call me a hippocratic Cat. It is important to understand that it might not be a define decision in the exponential vs. tractrix debate in case of a horn that works under 500Hz.  It might not be a final decision even in case if a person made a few different horns of different profile for the same installations.  The upperbass horn’ performances vary from so many different factors that it is very difficult to say where the specific sonic attributes came from a profile of the horn or from many other reasons (including the room coupling).

ulf wrote:

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.

Interesting. An oval horn would have less the off-axes imaging skewing of a rectangular horn but at the same time it will have mode gain form the boundary coupling. I do not see any benefits of MF oval, since MF horn hardly ever sit near floor but for an upperbass I think oval should be very good idea.

ulf wrote:

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.

I never tried this driver. BTW, if your throat allows and if you have some free time then fry my Fane Studio 8M driver. It would be interesting to learn what you think about it’s sound.

ulf wrote:

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!

… and you did a phenomenal job. The way in wich you’ve done it is exactly how it should be done.

ulf wrote:

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.

Hm, interesting. Can you expend on it? What exactly “was rather disappointing”? What changed subjectively with Sound when you applied the plug? I would NOT expect that a phase plug for a horn that woks under 500Hz might change Sound from “rather disappointing” to “acceptable”.  Would it be possible that the plug changed the throat resistance and “accidentally” changed the dumping of the driver’s cone? I need to ask” when you was tuning the size of your back chamber then you did it with the phase plug installed? One more thing I would like to dump on the table” the reactance of this specific driver to the plug induced throat resistance. Usually the upperbass drivers in horn disregard the presents of front chamber and juts “shot through”. If what you described was so then is anything in the TAD 1201 driver the makes it overly sensitive? I would particularly pay attention to the way in witch the dust cap is mounted. The dust cap generally is a big bitch…. Anyhow, I still would like to proposed that the diagram that you posted above might be true for higher frequencies but not for sub 500Hz. This why I asked you about your subjective sonic observations with and without the plug….

ulf wrote:

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).

I am sure that the upper bass horn with and without phase plug should have different low pass crossover. I would very much expect that if you bring via crossover the horn response with and without then plug to a common denominator then you might not see the “sweet spot” difference.

ulf wrote:

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.

Ulf, looking what you said I might presume that something is not kosher in your installation in the region of upperbass horn and MF horn integration. Regardless the phase plug is installed or not you must not be able to hear any HF attenuation FROM UPPERBASS CHANNEL when you move off axis that would be different from MF CHANNEL. The HF attenuation of course will take place but with upperbass and MF working TOGETHER should keep this “togetherness” off axis…

ulf wrote:

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.

Well, I do not question that the “group of people” heard change of sound. However, as I said above: by just sticking a plug in/out DOES NOT demonstrate the effect of the plug. When you apply the plug you need to change crossover and size of your back chamber in order to observe ONLY the contribution of the plug. Make simple experiments by measuring the changes in response of the horn and driver’s impedance when you apply the plug. I presume is I what it was what the “group of people” were hearing what you played with phase plugs. Though I might be mistaken of course but I have my reasons as this point do not ”buy” the benefits of phase plug for “under 500Hz”….

ulf wrote:

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.

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