DIY tractrix horn (Off Topic)

by Ivo, Thursday, March 02, 2006, 15:32 (4341 days ago)

Hi Bert and all,

I promised to show some pictures, when I made some progress.

A while ago, I made this trial version of 3 mm bending ply:
[img]http://www.v-2u.dds.nl/Hinten.jpg[/img]
I planned a square horn with smooth sides. The idea was that I cut the panel, glued them onto a frame and pulled them together with tie-rips. Once the horn was formed, I would glue the panels together, cut the tie-rips out and fill the holes before painting.

I got the calculations from a little piece of freeware software, called tractrix 12. It's intended to calculate polygon horns, like the 12-sided cardboard horn that is somewhere on the web. As you can see, it didn't give me the right result, so there are gaps. :sad:

I did new calculations and found the right panel shape, but this time I used matrial that was too thick and wouldn't bend enough... :blush:

I didn't get new material and saw it all over again. Instead, I decided to finish the first, failed attempt. I wanted to see if my design approach worked at all. So I tied the panels together anyway and used builder's foam to "glue" them together:
[img]http://www.v-2u.dds.nl/purhoorn.jpg[/img]
The idea is that it sticks like crazy and gets very hard, but is also foamy and damps the horn. Besides on the cracks, I also put foam on the entire panels of the horn, to maximize damping.

This is a front view:
[img]http://www.v-2u.dds.nl/purhoorn3.jpg[/img]
I cut away most of the tie-rips and it stays together. In fact, I dropped it on the floor, and kicked it around and it survives a small soccer match! As you can see, the gaps were filled with the foam, too. I decided it was good enough to try in my system.

I think it sounds alright. There is no obvious resonance, even though the plywood I used is very thin. The sound is clearly a bit warmer than another horn I have on the other channel, but that has obvious midrange and treble reflections and resonances that make it sound quite bright. The horn vibrates at lower midrange and bass frequencies

I'm not sure yet. Too soon to tell. The plan is to make two properly-designed horns now, using the approach, and listen to those for a while. Then I can form an opinion about their sound quality.

My initial, subjective opinion, is that the foam is pretty good, but that it would be better if I made the horns double-skinned and filled the space with sand. So foam the seams, but not the rest, probably 1-2 cm between the skins. Of course that's a lot more work, so something you do when you're finished "playing". This horn can be made in two evenings, one calculating and drawing, the other cutting and foaming.

That the story so far..
Ivo

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DIY tractrix horn

by p314 @, Friday, March 03, 2006, 21:09 (4340 days ago) @ Ivo

Hi Ivo,
what kind of "builders foam" do you use? I wonder whether I should do the same with the inside of my Audiovector replica horns and would like to know whether there are any points to take care of when doing this.

How heavy are the horns and what size do they have?
Regards,
Peter

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DIY tractrix horn

by Ivo, Saturday, March 04, 2006, 19:43 (4339 days ago) @ p314

Hi Peter,

what kind of "builders foam" do you use?

We call it PUR-foam, the can says isolation foam. It's sort of pale yellow.

I wonder whether I should do the
same with the inside of my Audiovector replica horns and would like to
know whether there are any points to take care of when doing this.

Well, I wouldn't. At least not just like that. An Audiovector is a beautiful horn and lots of work to make. Foaming them would be irreversible. It's makes a very good (or bad...) sticky connection to the horn. What if you don't like the results or would like to try something new?

Maybe you could put a plastic garbage bag in first and spray the foam into that. The foam will expand and still press against the horn walls, but if you want to remove it, it will come out cleanly.

I really hate irreversible mods on expensive or difficult things. :wink:

How heavy are the horns and what size do they have?

I don't know the exact weight, but it clearly weighs less than a 2 liter plastic bottle of milk, so less than 2 kilograms.

They are 61cm x 61cm, and about 40 cm deep, without the drivers. There is no backchamber, I like the balance as it is. I think I will cover the room walls behind with the same damping materials you would use in a backchamber.

Ivo

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DIY tractrix horn

by Ivo, Saturday, March 04, 2006, 19:50 (4339 days ago) @ Ivo

Hi Peter,

It's polyurethane foam. Which makes sense, considering the PUR in the Dutch colloquial name... :smile:

Ivo

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