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Old 7th February 2009, 04:23 PM   #1
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Default In Hornresp, need explanation of Atc and general guidance

Hi all. I'm trying to build an exponential horn that will look alot like the JBL 4550a (hopefully), but with three drivers. I suppose i'll stack the drivers (JBL 2226J's) one on top of the other. Drivers will run in parallel.

I have a two-fold problem:

1) I fail to understand what Atc means, as I have a problem with nomenclature like "normal to axis" in the help file.

2) I have a really sweet design in terms of the way the curves look.. i'll call its 3db down points 25hz and 1200hz. But the only way I've found to get the kind of response I want (usable above 500hz) is to set the Atc at 1600, Vtc at 14500, and S1 at 4600. Is it possible to build such a thing? I'm envisioning that the throat chamber would be shaped like an Isoceles Trapezoid, i.e. the long side of the trapezoid is the "mouth" (S1) where the transducers sit, being rather wide, and then it would slowly taper inward to produce a vertical slit at the end of the throat chamber.

Does this make any sense at all? Can anyone give me pointers in order to maintain that good high end response up to 1200 (or more) hertz?

Hornresp Parameters for reference:
Ang = 1xPI
Eg = 180 (yes, I intend to some day run that much power)
Rg = 0.02
S1 = 4600
S2 = 7000
Exp = 70
Sd = 692
Cms = 3.15E-03
Mmd = 98
Re = 10
Bl = 27.10
Rms = 0.13
Le = 1.75
Vrc = 700
Ap = 300
Vtc = 14500
Lrc = 15
Lpt = 0
Atc = 1600
I've omitted everything that's greyed out by the program, but the one relevant thing of course is the driver config, being Nd = 3P as I mentioned above.

Any help or guidance is appreciated!
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Old 8th February 2009, 06:23 AM   #2
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Default Re: In Hornresp, need explanation of Atc and general guidance

Quote:
Originally posted by letsbangout
I fail to understand what Atc means, as I have a problem with nomenclature like "normal to axis" in the help file.
Hi letsbangout,

"Normal to" is a mathematical / engineering term meaning "perpendicular to" or "at right angles to". As a general rule, most Hornresp users simply set Atc = Sd. For your proposed 3-driver design, that would make Atc = 3 x Sd.

BTW, some of your JBL 2226J driver parameter values appear to be rather strange :-). By my reckoning, according to the JBL specification sheet:

Sd = 880
Bl = 27.1
Cms = 1.58E-04
Rms = 5.04
Mmd = 85.19
Le = 3.5
Re = 10

Kind regards,

David
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Old 11th November 2009, 07:57 AM   #3
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I guess what I fail to understand is how Atc is any different than S1 in my application. I'm doing an exponential horn, a la JBL 4550a. Can you explain that a bit?

Also, I've found that unless I use a really high compression ratio (between S1 and Sd * n number of drivers), a la 2.5 or so, I can't get good frequency response up to 1khz. Per my original post, do you have any pointers regarding getting good response up to 1khz?

Also, any idea why I'm (apparently) getting some kind of strange resonance at 680hz? With the same JBL 2226J's, my parameters are now:

Ang: 1xPI
Eg: 180
Rg: 0.02
Cir: 0.44
S1: 1000
S2: 2800
Exp: 70
Sd: 880
Cms: 1.58E-04
Mmd: 85.18
Re: 10
Bl: 27.1
Rms: 5.04
Le: 3.5
Nd: 3P
Lrc: 25
Lpt: 0
Atc: 1000

Thx!
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Old 11th November 2009, 04:06 PM   #4
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diyAudio.com Wiki - projects by fanatics, for fanatics for general guidance on how to use HR.

For extended frequency response higher up, minimize Vtc (volume). Also you're using a high Le driver, you'll find mid and high range drivers generally have low Le. Use the directivity tool to see how the frequency response is, taking directivity into account.

Best regards Johan
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Old 11th November 2009, 05:13 PM   #5
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To be honest, good response up to 1k is going to be tough as drivers with that size a coil like being beamy. It's one thing to see a behavior in a simulator.

What's the adage? All wave guides are horns, but not all horns are waveguides. Depends how you shape the horn progression that hornresp gives so you don't screw-up the horizontal pattern.

Here's my advice based on my experience having made a mid-bass cab before that turned-out rather beamy. After you get your hornresp data for the path expansion, start with flat side walls expanding at not less than 50° from the center if you are going for a 90°H horn and use the top and bottom parts to fill the cross sectional area. This way, your horizontal pattern won't be too mucked up, but unfortunately my advice is mostly experimental experience based on prior failures.

With three vertically aligned drivers in a single horn, I don't know how you are going to get a good horizontal pattern.
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Old 11th November 2009, 06:16 PM   #6
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This looks OK. Use Ctrl+O to see combined response of horn+port. This shows a tractrix expansion rather than exponential which some people feel is more appropriate for use as a mid-bass horn.
Attached Files
File Type: txt midbass.txt (389 Bytes, 16 views)
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Old 11th November 2009, 07:29 PM   #7
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Hi there: I've been using Hornresp for tapped horns (TH) and have experienced resonances around 90-95 hz, untill I removed the back chamber (by setting s1 to 0.01 and the corresponding length c=0.01) and removing the loading chamber (Vtc=0 and Atc=0). These chambers seem to be modifing the response (SPL curve) by additional resonances which sometimes show up as hard to remove spikes or dips, requiring extream modifications to the basic dimensions to cancel. ....regards, Michael
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Old 12th November 2009, 07:29 AM   #8
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Quote:
Originally Posted by Rademakers View Post
diyAudio.com Wiki - projects by fanatics, for fanatics for general guidance on how to use HR.
Thanks.. my reading of Atc now is that it's the cross-sectional area of the throat chamber, whereas throat chamber is defined as the portion of the throat prior to reaching the beginning of the horn segment (although I didn't see that broken down anywhere in the wiki, that's my interpolation..)

Quote:
For extended frequency response higher up, minimize Vtc (volume).
I've tried that before, it seemed to have minimal effect. I'll give it another whirl now that I have a better handle on the parameters.

Quote:
Also you're using a high Le driver, you'll find mid and high range drivers generally have low Le. Use the directivity tool to see how the frequency response is, taking directivity into account.
While that may or may not be true (I wouldn't know), in the case of this particular driver, the Le is high(er) because the motor structure is literally double that of the 8 ohm 2226H. Double the wire, double the impedance, double the Le.
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Old 12th November 2009, 07:38 AM   #9
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Quote:
Originally Posted by davygrvy View Post
This looks OK. Use Ctrl+O to see combined response of horn+port. This shows a tractrix expansion rather than exponential which some people feel is more appropriate for use as a mid-bass horn.
I plugged that in and found that your bass response is much worse than mine (my 3db down is like 27hz on the low side, assuming a baseline of 135db, yours is something like 32hz). Also the phase response from your design is _horrendous_. And there's still the same strange resonance at 600hz. But thanks for the contribution. I'll be sure to post my final parameters back to the forum, once I've finished redesigning.

Also in relation to what you said about vertical pattern control, that's a very valid point. One thing I could do is simply not have a vertical flare - essentially repeating the Altec 817 (which, by the way, I'm thinking about modeling since their plans are available online). Also, I don't suppose there's any advantage to flaring the vertical down more narrow, right? Would probably mess up the phasing tremendously, not to mention, hornresp might not be able to account for any of that...
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Old 12th November 2009, 09:39 AM   #10
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The shape of the horn that HR assumes is a cone, IINM. Better ask for a confirm. I, myself, wouldn't believe any of the polar results once I deviate from a cone shape. Hornresp is for HORNS, not wave guides. HR knows nothing about the shape of your progression.

I was assuming you we're using this for mid-bass not caring about 70 and below. The phase response is funny because of the port to control displacement. If you don't want a port, you may do so. Also note the port placement is a relative thing for the sim and not stored in the data. Placement can have a wide change in response shape as you'll discover. If you really do want to push it to 130Vrms across each driver, you'd better respect the displacement graph. Your data said Nd=3P, not Nd=3S.

Oh, and I said HORIZONTAL pattern, not vertical. Is this for a home or a PA? Personally, it doesn't make sense to me to push the range as far as you are for a PA for numerous reasons I won't begin to list.

Oh... And this is really fun... A horn (wave guide) that has a well controlled coverage pattern over it's frequency range is never flat without outside assistance I love that one! Just look at those EV constant directivity horns, they ain't flat.

Please test your horizontal pattern with a cheapy plywood test build before committing to the nice furniture grade wood you'll probably be using. I've been down this road.

Sims are just a tool out of a wide range of design tools you may want to use. Don't be surprised at how correct HR is in some things, and how wrong it may be in others due to assumptions it is making that don't hold true with what gets built. Make some sawdust and verify, please. Flaring only the sides, doesn't mean the top-end is going to stick to it and follow. For example, look at the old late 70s 120 horns. They all have flat sides with the flare on the top and bottom. Also, if you are off-axis at 45 and looking at the cone, and can't see it, you certainly aren't going to hear 1kHz from it.

I look forward to pictures
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