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imperial project questions (Read 1846 times)
Sean
Ex Member



imperial project questions
01/07/06 at 18:04:53
 
I have two Electro-Voice EVX-180B cone drivers, and two JBL 2441 2" compression drivers, which I originally intended to use in a two-way system as per Steve D's original imperial project.  I have a couple of questions about this, though:

The mouth of the HF horn built by Steve and a few others, at least from the posted photos, appears to be as wide as the imperial cabinet itself.  This presumably allows a very low cutoff for the HF horn, but I'm curious as to what effect this has on HF rolloff?  What about efficiency?  I'm guessing that this profile is tractrix adjusted for area since the upper and lower faces of the horn are not parallel?  40 degree dispersion, or something else?  

The 2441 specs indicate that it plays to 18khz.  Do I need to add a driver to take the reponse to 20k?  Or is this unnecessary?  Probably for my hearing, but maybe kids would hear a difference?

Next, highest recommended crossover for the EVX-180B is 500Hz, and the lowest recommended crossover for the 2441 is 500 Hz.  I don't know what the actual response will be, but I'm guessing that if I'm forced to cross over at 500, I'm too close to the limits on both drivers, and should add a midrange driver to the system?  In that case, I was looking at the JBL pro drivers - specifically the 2012H 10" cone driver.  The only reasonable way I can see of integrating this into the imperial cabinet is to place the EVX on one side of the baffle, and the 2012H on the other.  This raises a couple of concerns:

1)  What will this do to soundstaging?  Not a huge issue with PA cabs I know - but in reading Steve's original imperial thread - I don't want the "head in a vise" problem.  (incidentally, how close can you listen to these cabs?  Near field?) Of course, I'm not going to be running full range (Behringer DCX2496/DEQ2496 combo), but still, there is an effect created by having the mid and high sources at different locations - how significant is this?

2) Will the imperial cab function properly with the LF driver on one side of the baffle, or do I need to change the way that the rear chamber opens into the horn with this arrangement?  Since the mid driver would have its own small sealed enclosure, taking volume out of the LF rear chamber, I would need to do this carefully so I don't totally screw up the throat geometry in a way that negatively impacts the LF driver.  Thoughts?

3)  The thought crossed my mind that it would be cool to horn-load the 2012H as well, so that I have a three way cab with all drivers in horns.  I couldn't figure out a way to do this in the available space on one side of the imperial cabinet that didn't make it look like a carpentry nightmare, so I shelved that idea, but still wonder if there's a better way?

I think my understanding of efficiency is severely limited.  The efficiency specs given on the driver spec sheets will allow me to calculate SPL in free air or OB, correct?  How do I determine what effect the horn lens has on SPL?  How do I determine how many drivers I need for a balanced system?  (in this case, I was curious because an imperial with an 18" and a 10" looks kind of funny, but with two 10", looks much more intimidating, and then I realized that I have absolutely no idea what I actually need to get comparable SPLs).  How do I figure out the total combined SPL of the system across the entire frequency range?

Finally, as I had originally intended to build one of these with the two EVX's in it, I scaled the plan by 18/15 - quite a massive cabinet.  If I go down to only one LF driver, does the horn cutoff actually change (i.e. should go back to original dimensions), or does this only affect SPL?

Sorry for all the questions, but I'd like to get this nailed down before I start spending money and making sawdust.

-Sean
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DirtDawg
Ex Member



Re: imperial project questions
Reply #1 - 01/07/06 at 19:07:32
 
Uh ..... What did you say? Smiley

That's a lot of questions and you will need a lot of responders to answer them all.

I can tell you that I have 2 - 2441 (aluminum) and several 2445 (titanium diaphram) drivers and I've used them on larger horns at 300Hz XO before with half power and they are not stressed at all. I crossed over to bullets above 8kHz and the response was very flat. The HF response was compromised a little with the larger horns, mainly in dispersion. To get 15kHz to widely disperse requires a totally different horn than 1kHz. Most horns used for a wide range are a compromise between low frequency cut-off and hi frequency dispersion. Don't worry too much about the 2441s being used at their limits. They will easily excede their specified ratings as long as the power is not clipping.

Rather than adding another 10" mid I would recommend another 15" and don't scale anything down. As far as near field, you are looking at 8 to 10 feet away preferably, more would be better.

Quote:
The efficiency specs given on the driver spec sheets will allow me to calculate SPL in free air or OB, correct?

Most efficiency specs are rated with the driver in the proper/recommended cabinet and the cab in free air. How they get their free air is not always predictable. A cabinet on a pole 6' in the air with a microphone 1 meter on axis above that is what many do. I think I could make an argument that the ground would reflect at 6 to 8dB below the direct signal, especially at low frequencies, and so it's not "free air", but that's how many manufacturers do it.

Unfortunately, the HF compression drivers are often loaded into a straight pipe the same diameter as the exit and measured at different places, usually 6' down the pipe. This often skews the efficiency by +6dB. That's fine as long as when you compare 2 drivers' ratings they are done the same way.

Quote:
Finally, as I had originally intended to build one of these with the two EVX's in it, I scaled the plan by 18/15 - quite a massive cabinet.  If I go down to only one LF driver, does the horn cutoff actually change (i.e. should go back to original dimensions), or does this only affect SPL?


Don't change the plans. If you scale anything then everything changes, including frequency response, low cut-off, and efficiency. You might get lucky, but I doubt it.
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« Last Edit: 01/07/06 at 19:11:17 by DirtDawg »  
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stvcmty
Ex Member



Re: imperial project questions
Reply #2 - 01/07/06 at 19:38:31
 
In regards to your 500hz crossover question:

Try to your speaker full range.  If it doesn’t show breakup or a harshness, then drive it directly.  By running the low element full range, you can use a simpler crossover for the high frequency element, and a slightly higher crossover frequency.

It has been my experience that beaming is not a serious problem on 15” speakers run full range, if the crossover to the high frequency element is handled right.  The dispersion and directional nature, as well as higher efficiency of HF drivers leads to any beaming being drowned out.

When I run a speaker full range, I use a simple crossover for the HF element; resistor, a capacitor, and the element all in series.

Below is a diagram shown for an imperial with 2 speakers:



Best of luck,

Stv
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Sean
Ex Member



Re: imperial project questions
Reply #3 - 01/07/06 at 20:13:27
 
The EVX's are 18's, not 15's.  Also, I will be bi or tri amping, with active crossover(s) which can be set wherever I like.
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DirtDawg
Ex Member



Re: imperial project questions
Reply #4 - 01/07/06 at 20:37:22
 
[quote author=Sean  link=1136660693/0#3 date=1136664807]The EVX's are 18's, not 15's.  Also, I will be bi or tri amping, with active crossover(s) which can be set wherever I like.
[/quote]
YEP, sorry. My fault.
With an active you will be able to set the XO much lower because of the greater slope. I forgot to mention that when I used the 2441 at 300Hz on a much larger horn, I was using a 24dB/octave roll-off. If you try the cap only on the HF/full range LF idea make sure you go above 1kHz XO with the cap size to be safe.
A capacitor alone will roll-off at 6dB/octave. One reason you won't have as much problems with beaming is that a cap will shift the phase by 90 degrees and some interesting canceling at the overlap frequencies will occur. You might like the effect.
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« Last Edit: 01/07/06 at 20:38:48 by DirtDawg »  
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