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 Subiectul mesajului: Horn Flares
MesajScris: Dum Ian 08, 2006 11:47 pm 
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I've been trying to design midrange and HF horn flares to be built out of fibreglass. The midrange driver, using an 8PE21 as per Xtro is causing me some headaches, mainly regarding the throat/phase plug section. The idea is to use a circular or elipsoid conical horn with the Peavey designed quadratic throat (http://aa.peavey.com/downloads/pdf/qwp1.pdf). This would be simple with a non-phase plugged horn and should work well on the 1" HF but it's a bit more tricky to maintain the flare rate with a phase plug in place.

Another thing I'm worrying about is HF beaming. With no phase plug, if the horn has an length of 30cm and a 60deg dispersion, the curved section will extend halfway down the horn before becoming conical. Could this cause beaming at the upper end of the horns range? Adding a phase plug may then require a section with parallel outer walls, I'm thinking that's not such a good idea, even with a phase plug in the centre. Having said that, the midrange on the air motion seems to do something similar. Without having intended to do so, it does seem like I'm trying to do something very similar to the Air Motion mid and top. At least we know that sounds good, so can't be a bad way to go!

I will try to do some drawings to post to make it easier to describe where I've got and where I've hit problems.

Construction wise, when I can get some foam to make moulds out of, I might start with a copy of the Xtro horn in fibreglass to use as a benchmark for any new designs.


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 Subiectul mesajului:
MesajScris: Mar Ian 10, 2006 1:39 pm 
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i've got something a little easier to build for my prototype, for the hf i am trying to emulate the res5 elements, that is to say a square based pyramid type shape with a bit of filler in all the corners and mouth area. i'll be using wood, i hate fibreglass with a passion. on the subject of wood versus fibreglass, why on earth would you build an x-tro horn out of fibreglass when the shape permits relatively simple construction from timber? personal preferences showing through here.

i was also thinking of trying a variety of different plugs in a conical walled horn so as to avoid the curved outer edges at any point, i'm really worried about the beaming problem since having had a good listen to arrays of my mids at different crossover frequencies (although i am far from having "scienced" it). this keeps construction of the outer horn quite simple, only the phase plug may become quite complex.

i can't decide if it's worth building a few horns for my mckenzie 8"s to get a feel for the physics of the matter, but i would rather use a smaller format neo driver in the final design (particularly interested in the 18sound).

james.

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 Subiectul mesajului:
MesajScris: Mar Ian 10, 2006 11:20 pm 
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One reason for building an Xtro horn out of fibreglass is to get some practice before trying something more complicated! I've never actually worked with fibreglass but having looked into it and talked to people who have, it seems like the logical choice, especially if I want to make multiple cabs. When I said a copy of an Xtro horn, it probably won't be exactly, but I will start with one without a phase plug to see how high I can get without. I'm also planning to make a fibreglass rear chamber which fits round rather than over the magnet. This makes it possible to get the rear chamber down to sub 1litre and keep the voice coil vent in free air.

From what I've done so far, I don't think it's possible to add a phase plug to a straight walled conical horn without the phase plug expanding larger than its diameter at the throat or introducing an initial flare rate greater than the main conical section. The only exception would be a phase plug which extends the entire length of the horn. Either a straight, or constricting outer horn section seems to be required for a phase plug.

I've also considered using a 6" driver, such as the 18Sound one which looks very nice but for the time being the 8PE21 is a known quantity and easily obtainable for a good price (especially if you work for a B&C dealer :wink:).

It's all interesting but I seem to be spending a lot of time tearing my hair out (what's left of it!) over the trignometry for quadratic throat expansions. The sooner I can get my head round it and create a spreadsheet the better!


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 Subiectul mesajului:
MesajScris: Mar Ian 10, 2006 11:29 pm 
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ceharden scrie:
One reason for building an Xtro horn out of fibreglass is to get some practice before trying something more complicated! I've never actually worked with fibreglass but having looked into it and talked to people who have, it seems like the logical choice, especially if I want to make multiple cabs. When I said a copy of an Xtro horn, it probably won't be exactly, but I will start with one without a phase plug to see how high I can get without. I'm also planning to make a fibreglass rear chamber which fits round rather than over the magnet. This makes it possible to get the rear chamber down to sub 1litre and keep the voice coil vent in free air.



be advised that the spider of a speaker is not a sealant, so by venting the magnetic structure in free air, you are infact unsealing the rear chamber of the horn
the air will pass trough the spider, between the voice coil and polar piece of the magnet, and finaly exiting trough the pole piece vent.

in my opinion not a verry good ideea... :roll:

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 Subiectul mesajului:
MesajScris: Mar Ian 10, 2006 11:38 pm 
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I had considered that issue but since I'd seen it done on an EAW 10" midrange horn (Dance Club series) I assumed that it would not be a practical problem. Mainly since the air would have to go through a very narrow folded path round the voice coil. Do you have practical experience with this? Given the very small rear chamber required, overheating might otherwise be a problem. It's either that or use a heatsink mounted in the rear chamber.


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 Subiectul mesajului:
MesajScris: Mar Ian 10, 2006 11:41 pm 
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I did not see this in practic, But if the spider could be made sealant, the cooling of the voice coil could be extreme in this configuration

Try and look if the EAW 10" midrange did not have a sealed spider assembly 8)

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ing. Mircea Bartic
Director general

Poweraudio Romania
Drumul Tarpiului, Nr.30, Bistrita, Jud. Bistrita-Nasaud
Telefon: 0766332366
E-mail: office@poweraudio.ro
www.poweraudio.ro
facebook.com/poweraudio.romania


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 Subiectul mesajului:
MesajScris: Vin Ian 13, 2006 1:40 am 
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Finally I'm getting some results with my Matlab scripts. I can now calculate and plot horn flares, albeit in 2D only. 3D plots will happen!

Here's where I am so far:

Imagine

Hopefully this should reduce the amount of drawing on squared paper I'm doing to calculate flare expansions. One very useful thing is the ability to put in an abitrary shaped phase plug and the outer walls will adjust to keep the area consistant with the desired expansion.


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 Subiectul mesajului: matlab...
MesajScris: Vin Ian 13, 2006 2:37 am 
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i tried using matlab to do the reconstruction algorithm for an electrical impedance tomography system (university dissertation), i confess i never really got my head around it and ended up doing the whole thing in a very large and very unwieldy excel program.

not surprisingly i have opted for billy boy's wonder for this project, ceharden you must show me how matlab works if we ever get together.

james.

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 Subiectul mesajului:
MesajScris: Vin Ian 13, 2006 10:51 am 
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I started trying to do it in Excel but realised that I would soon get fed up with its limitations. I'm no Matlab expert but have used it for a few different things at work/uni so I can do basic stuff.

What do you think of that horn flare? Does it look like it'll beam badly at HF? I reckon as it's a curve rather than a sudden change in flare angle (eg MT122) that the beaming will be less severe although some is probably unavoidable.

Have you looked at throat areas for the 8PE21? The Xtro uses a fairly large 144cm2 throat, probably to avoid path difference cancelations. Hornresp seems to suggest that even more efficiency can be obtained by reducing it even as far as 50cm2. Such a horn driver should work well with a high compression ratio you would think? To get a throat that small, a phase plug is probably essential.


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 Subiectul mesajului:
MesajScris: Vin Ian 13, 2006 11:52 am 
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that is half the reason for having a phase plug i reckon, so yes. i'm not so sure about the hf beaming problem, instinctively from just looking at the picture i would have thought it would be a real problem as although there is an initial expansion made up by the phase plug, the outer walls actually completely reverse direction. as i said though, there is no scientific basis for this premise and in fact that idea may well work perfectly.

in order to keep my truncated square based pyramid thing's straight walls everything gets very messy up at the throat end, i think i really need to start thinking in 3d about the volumes involved. the 8pe21 will love the tiny front chamber i reckon, path length differences is the obvious reason for the x-tro's throat area but perhaps walt might comment? i do worry that our mid-bass will struggle to keep up if we make the high mid any more efficient, it may mean opting for something a little more hardcore (read expensive) than the sn12b.

james.

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 Subiectul mesajului:
MesajScris: Vin Ian 13, 2006 8:29 pm 
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Regarding the walls reversing direction, if you look at Walt's section on phase plugs, most of them incorporate such a feature in order to maintain the correct flare rate. The key to avoiding HF beaming is probably to get from the throat to the conical flare as fast as possible. I might move away from the quadratic throat slightly in order to acheive this. HF beaming is probably something we will have to accept and try to mitigate rather than eliminate. The narrower the dispersion of the box the less it will be a problem I expect.

Making the hi-mid more sensitive can't be a bad thing. That section will be handling a higher bandwidth than any other (300-3kHz) which is over 3 octaves. We want to be running the driver within its limits rather than close to or over.

Nexus3 and I both initally considered a twin 15" but that would probably make the box a bit on the big side. You are right that the mid-bass will be the limiting factor but if you cross over to the subs at 100Hz or so, they only have to do 1.5 octaves so we might be ok.


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 Subiectul mesajului:
MesajScris: Lun Ian 16, 2006 11:09 pm 
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be advised that the spider of a speaker is not a sealant, so by venting the magnetic structure in free air, you are infact unsealing the rear chamber of the horn
the air will pass trough the spider, between the voice coil and polar piece of the magnet, and finaly exiting trough the pole piece vent.

in my opinion not a verry good ideea...


I've discussed this with a couple of people and while no-one had a definite answer the conclusion seems to be that the effect of the vent will not be significant. Guess I'll have to try it.


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