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3rd June 2006, 02:39 PM  #1 
diyAudio Member
Join Date: Sep 2005

Back loaded big spiral horn with Fostex FE206E
Here is a project I'm going to begin. I wanted to try a horn with a "nautilus" shape, but it's like nobody has tried this before (B&W nautilus is based on another concept).
Advantages of this design is that there are no angles, no parrallel surfaces and a very big mouth. Besides it is a "natural" shape. See the picture attached: it is what I want to build scaled 1:20 This is a stright exponential horn empirically folded as a spiral. The circle at the end of spiral is the driver. The horn is designed to be appended to the wall. Dimensions are quite big: positioned like shown in the picture (mouth line at 45°) it is 250 cm high, 170 cm wide and 60 cm depth (just the first 100 cm from mouth, then goes decreasing to a few cm). A couple of these may fit in a normal free wall 4 m wide. Having more mathemathical knowledge (that I have not ) would be nice to drow a logarithmic spiral matching the parametters shown below. Somebody maybe can do it? Parameters: Flare freq. : 35 hz Troath area: 70 cm2 (calculated on Fostex FE206E) reduction ratio: 4 (2 walls) Horn lenght: 420 cm. Mouth area : 15039.37 cm2 Mouth section: 60 X 250 cm Mouth lenght: 250 cm (~ a quarter wave lenght of 35hz) Although I've already built some enclosures, it's the first thime I do it from scratch. So I wonder how such big things will sound. Thomas 
3rd June 2006, 07:22 PM  #2 
diyAudio Member
Join Date: Aug 2003
Location: Mass.

That's a pretty cool looking design with a VERY low WAF. You must be somebody who isn't married.
I'm not sure what you mean by a "logarithmic spiral". Could explain? Denis 
3rd June 2006, 09:08 PM  #3 
diyAudio Member
Join Date: Sep 2005

Logarithmic spirals:
a spiral is a curve turning around a fixed point growing its distance from that point at each turn. In a logarithmic spiral this progression is geometrical and is described with a logarithmic formula. This formula for the curve growth may be the SAME as that commonly used to calculate a normal exponential contour for a straight horn. For my empirical spiral I used an exponential horn formula then turned the contour in a spiral shape with a graphic software. Should be interesting to have a formula for a spiral horn contour starting from the throath lenght and growing till the mouth lenght at the right distance. Wife factor: Yes the horns are big, but they take much less space, in a room, than normal speackers on a stand. The stand must be away from walls then occupies at less one square meter of floor each. My horns are to be fixed to the wall. Their depth are of 10 to 20 cm from ground to 180 cm. The floor is free. The mouth is against 2 walls, instead of corner: the mouth is bigger, but there is no need to place them away from back wall. Yes, they occupy the whole wall, but I think they are much cutier, if well finished, then usual big boxes. All the cases, they are for my room (a normal 4x4 m) dedicated to sound listening. Ciao Thomas 
3rd June 2006, 09:17 PM  #4 
diyAudio Member
Join Date: Aug 2003
Location: Mass.

Hi Thomas,
I would think your original spiral is better as it represent a coiled exponential horn. You second spiral looks like a coiled conical horn, although you probably could adjust the formula to draw a coil with any expansion you want. Regards, Denis 
3rd June 2006, 10:59 PM  #5 
diyAudio Member
Join Date: Sep 2005

Hi Denis
The coiled exponential horn is what I call "empirical" home made logarithmic spiral. Of course there must be a formula to generate my horn with a logarithmic spiral (but what?), because the growth of both, the straight exponential and spiral, are logarithmic (the last spiral I posted too). The pure logarithmic spiral shape is a bit different, but, I guess, better, because is the shape used by the nature to fold long things (cyclones, galaxies, spider webs, nutshells..), and is new. Do you think it will sound? What about the thickness of wood necessary for these big surfaces? Ciao Thomas 
4th June 2006, 07:05 AM  #6 
frugalphile(tm)
diyAudio Moderator

This is not a new idea, althou usually it is fitted to a rectangular box (Fostex even has a version for the old FE108Sigma) ... probably the most elegant execution is the Cornu horn.
dave
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4th June 2006, 08:29 AM  #7 
diyAudio Member
Join Date: Sep 2005

Hi Dave
From Cornu horns I lent the idea to calculate a straight exponential horn and coil it. But the concept is different: Cornu horn put the driver on the horns mouth, like B&W Nautilus (see picture attached). My concept is to have an uncompromised stright exponential horn (420 cm long and 1/4 fc wide mouth) without angles. The guess is that coiling the horn in a spiral has same sound than holding it stright. Ciao Thomas 
4th June 2006, 08:44 AM  #8 
diyAudio Member
Join Date: Sep 2005

FE108Sigma spiral
Hi Dave
I've found now the discontinued Fostex FE108Sigma recommended enclosure (see picture attached). Yes, that is the concept. But it is a usual folded horn with square angles that affect sound speed. Have you other link? Thanks in advance. Thomas 
4th June 2006, 03:04 PM  #9 
diyAudio Member
Join Date: Sep 2005

Something like this
Here attached is a scaled model of the spiral horn.
Somebody has an idea about how this horn will sound? Regards, Thomas 
4th June 2006, 03:10 PM  #10 
diyAudio Member
Join Date: Dec 2004
Location: Eichstetten

The folding might be good, but I see a problem with the placement of the driver. It's placed in somthing like a tube which will give you nasty resonances and early reflections in the midrange....

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