Hi Steve,
Welcome to the Forum!
That is one amazing speaker you got there! I am currently building a pair myself using Steves plans.
Note:
Some of us forum users have slooooow Dial up connections and it takes a long time to load such large pictures such as the Imperial you just posted. Please size with this in mind. Thanks and again welcome to the Forum      Darrel

Thanks guys.
I'm going through another one of those phases with hi-fi.
This happened after listening to a set of $10,000 Martin Logan electrostatics and a $100,000 Avante Garde Horn system.

...opinion section starts
I was completely disappointed in both of them for various reasons.

a) ribbon tweeters are better than horns or electrostats
b) none of the low frequency components in either
system was worth a rat's ass.
c) the horns had major problems with phase and blaring
d) the electrostats were harsh in the upper mids

This is strange to say this, but maybe not, but I was simply
disinterested in such exotic hi-fi systems.
All this expensive cabling and amplifiers and DACs...and yet
they did not get the speakers "right".
And speakers are 90% of the game!!!
...OK, opinion section over

I spoke recently with Magnepan and the owner's son,
Steve Winey, who is a great guy and took the time to talk
about the problems I was having and it kind of charged me
up to get moving.
...because I've recently repaired an original pair of
Magnepan Tympani Bass Panels.

Each speaker is hinged with 2 panels.
Each panel(4 total) is 6foot by 16 inches.
I'm going to use these panels as mid-bass for 100Hz to ~400-450Hz.
My present system has 2 transmission lines with 2 JBL 2242 transducers(18"-800W). But it is not as good as the horn I built. Not as fast and cannot match the SPL level of a horn.
I built them because...well...I wanted to figure out the technology.
But thank God I'm over that.

I replaced my Magnepan 3.5s with the Bass panels just to
hear the bass and I am ashamed to admit I nearly started crying. Kettle drums, tympani, kick drums tom toms...all
with attack, real volume and authority.

But the transmission lines just could not keep up with the mid-bass panels or compete for sheer gripping power.
So, I think I'm going to build a 6 foot version...maybe using both drivers together as the sub-woofer! I dunno.
I downloaded Steve's plans but at first blush I couldn't understand how he modified the throat.
I know that any sub design can be scaled and I know
the "size" of the throat, which is the crucial part of all this.
There are many "horn" designs out there and they can be
folded any way...it doesn'y matter for a sub...but finding good
theory on the throat which is so critical is scant.
I found an interesting horn made especially for the jbl 2242
by jbl called the ASH6118.
Here's a pic here:
http://www.steevee.com/images/ash6118.jpg

It's similar actually to the Jensen Imperial...
Enjoy!

Nice pictures.
What about measure for a 18' in jensen imperial ?
How must it be reduced ?
Thank's,
Marc

[quote author=footstony  link=1072893817/0#6 date=1097546065]Hi Steve

Welcome to the forum. You have the biggest horn.

I have posted some calculations for throat size in "1/2 scale imperial" post on this forum.

I made a half scale version of the imperial and found that the horn doesn't work unless the throat size and compession chamber is right for the speaker you are using.  You can use the exponetial/hyperbolic calculater on the single driver website at http://melhuish.org/audio/exphypcalc.html  to calculate any extention/reduction of  horn length to correct for throat size.
For a full size imperial horn use m=0.7 and flare frequency=40Hz

Regards Philip [/quote]

Sorry...haven't been here for quite a while...miss this place!
Phillip, you are correct for the reasons mentioned. But you scaled it down which limits low frequency output, power, etc. Simply scaling down may walk you into a territory with an undersized throat and this may also cause more problems than a slightly oversized throat(which if it is...is a lot easier to adjust if needed!)...at least this is what I've "heard" when listening to horn subs. I did the measurements for the Hartley woofer and found it pretty darn close when scaled up. Anyway the proof is in the listening and it sounded unbelievably good and the room measurements bore it out. The response was flat down to 30Hz and it started to tail down gradually. And the efficiency, dynamics and raw output...live music sound levels, well it was a successful ride to say the least. I think it sounds miles above the original 15" cabs ...no comparison IMHO. Also, check out the Hartley sub-woofer...It truly is one of the great speaker designs of all time.

Cheers!