Ever think of building a Cornu Spiral horn? Now you can!

Hello Cornu enthusiasts.
I am back to designing a small CNC cornu implementation and am trying to add to my know-how, but doubt I will get to the levels of expertise some of you seem to have. I am designing in 3D and am mainly guided by aesthetics and elegance of construction.
I will post some pics here when I get a little further.
My question for today is about the size of the chamber in which the driver sits - the "hub" of the spirals. What is the impact on the sound if this is bigger or smaller in volume? This chamber is kind of a plenum which feeds each spiral horn.

The little design image at the very start of this thread..
1707941122409.png


shows the inner square box being significantly larger than the 10cm driver. My design will be curved all the way to the end, not square, and I can either make the ends of each spiral closer to the driver (making the chamber smaller and the horns a bit longer) or I can make this inner plenum space bigger than the driver like in the original sketch above, so there is more room to build it nicely and maybe more breathing room for the driver which might be beneficial.

If I make it as small as possible the open end of the spirals will get compressed together, which is harder to build and fit around the driver, but the horns themselves can be a bit longer (more turns), so I am not sure which is best.
I also have read here about stuffing this chamber with foam or dound absorbing material, but don't know much about how this affects things either.

I will try to incorporate any tips I learn here and share a WIP design with these tips implemented and see what the group thinks.
My horns are all individual end to end and do not split into two like in the original. So, if I am understanding these single horns could be calculated with Hornresp or other software if I provided a CAD file of it to someone willing to run the calcs, but I am not quite to that point yet.
 
I've no idea what the FR of the horn part is; is it just "bass" or do mids come out there also?

What I find irksome about the design is the immediacy of that back wall to the driver. I've seen many speaker designs that attempt to "treat" the inevitable hard stop to the sound emminating out of the back of the driver - in an enclosure at least. On a scale, I'd envision this "right up against" design at one end, Open Baffle on the other.

Let's say you put a stuffed "traffic cone" behind the driver - then the speaker loses its advantage of being able to be hung on a wall like a painting. Some stuffing sandwiched between metal screens on the back surface would be flatter, but then sound would leak out the back of the speaker, which probably would ruin the whole idea also in more ways than one.

It's an aspect to think about. As long as you're in 3D CAD, one would think there'd be a way to smoothly curve the horn entries upwards towards the back of the driver - looking into the cavity, it'd be like radial fins with smooth curves up to a radius, about the size of the speaker magnet diameter. This means a Neo magnet structure, being much smaller, would give more room to play with the shapes and form of that area.

The design of the horn entry of course would need to match the back of whatever driver is chosen, which could have anything from holes in a stamped metal basket to posts in cast plastic. Since no speaker I know of was designed to accommodate horn entries at its basket, one would have to cleverly choose among what's a given there, to adapt it to such an idea as a "fitted" horn entry Cornu.
 
Thanks for the reply. The design I am tweaking looks like this -
1707945077423.png

this is with the front baffle hidden so I can see how the spirals approach the driver. You can see the spiral hit the driver jus below the center of this screen shot. This model is a rough approximation of the Mark Audio Pluvia 7.2.
I can either trim the corner off where it conflicts, or make the inntermost spiral radius about 1/4" bigger so it does not hit, but also does not need to be trimmed. This can only really be understood when I have this built and have the driver to fit around
I am still not clear on how to balance the size of the plenum behind the driver vs the width of the openings to each horn, and the slightly longer horn length which making this plenum smaller would achieve.

Here is a slightly wide camera angle on the same design -

1707945458405.png
 
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There is an Akabak script for this horn posted in this thread. It is quite comprehensive and the chamber volume is one of the variables. In the scheme of things, given how much variation in the midbass the horn has, although very musical, it’s no “monitor” - the chamber depth is a minor effect. That is, it is not a key driver in the design that impacts the sound. The overall length of the channels and choice of driver is more important. The fact that I have made this by varying the depth quite a bit - and that varies the chamber volume, yet it still works for a bunch of different drivers. So really, it’s not that sensitive to the chamber volume. Don’t make it so big it becomes a reflex horn. Regarding the wall behind the driver, you can add an internal cone to prevent that direct standing wave or add a layer of melamine foam. I have not found this to sound “boxy” at all. Most importantly, make a prototype in a few nights in the kitchen table with foam core to test your design before committing to cutting wood.
 
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You need to install Akabak - (Acoustic Abacus) software. It’s not an easy software to learn but very powerful and about the only one that can handle a horn with 4 mouths. Difficult but all you need is to run the pre made script and vary the chamber volume to see the effect.

There is a thread for Akabak on DIYA.

https://www.diyaudio.com/community/threads/akabak-3.353871/

Here is the Cornu script written by Don Hills:
https://www.diyaudio.com/community/...peaker-enclosures.223313/page-27#post-3276695

Change the Depth - and that varies the chamber volume and horn volume. You will see the effect.
 
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Colin,

Forget the plaster, noit stiff enuff. And translam is a HUGE waste here (even more than usual). 3D print it. some sort of skins overtop of a fill such as a bucky-like tetrahedron structure. Acyrilic is fine for a baffle. It is really well braced so does not need to be that thick. Likely a big contributor to total budget.

BTW: answer mfrom reading your email not your post/ I might have more i am abit under the weather ATM.

I've no idea what the FR of the horn part is; is it just "bass" or do mids come out there also?

To approximate, take the circumference of the mouth> In this case, 4 x 2x(33 cm + 10 cm) (guess) = 344 cm. Multiply by 2 since it is wall mounted = 688 cm = wavelength of 50 Hz, below that it is a TL.

Colin: the size of the coupling chamber can be roughly determined using the formula in Dinsdale. Since you are fine tunging the design — i applaudf that — this is a good basik primer on horn design with theoretical formulas.

http://p10hifi.net/TLS/downloads/Dinsdale-Horns-3parts.pdf

dave
 
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