There have been several people who've popped in on this forum mentioning that they've scaled the Imperial up or down with good results without really discussing how to go about it.  That's left those of us who want to try something smaller a little afraid to try because we want some comfort regarding good results.

I've spent a lot of time recently modeling the Imperial and different scaling with different drivers.  Here's what I've found including some basics about horns for those who are like I was not too long ago:

The Imperial is a rear horn designed for a single 15" driver with the big cabinet built into a wall.  It has a very big compression chamber, which enables the deep extension (like a big box vented sub), a large throat in relation to the driver giving it high efficiency, a horn length of about 2 meters, and a big mouth (necessary to reduce distortion in the low frequencies).  Any full height and depth Imperial variation (horn length and expansion remain the same) results in a maximum response at between 80 & 90 hz and it drops off like a rock above that.  From that point up the sound comes from the driver's front.  What happens below that point is driver dependent with some flexibility resulting from reducing chamber volume, but the cab is trying to play 20hz with a much shallower rolloff than the high frequency cutoff.  

Big strong 15's with low Qts, High BL, low vas, Fs 40 or more , fair the best.  These get you 100db efficiency down below 30hz (20hz in a corner) and a smooth response up to the 105db peak at about 80hz.

What I wanted was a full range or coaxial driver that would work, so I didn't have to build the horn tweeter like Steve did.
Unfortunately my 15" Selenium coaxials just don't work.  I couldn't get their 12" version to model well either, due to the Qts of over .50 for both drivers.

You need high efficiency drivers because they need to keep up with the very efficient horn, especially near the acoustical crossover point.

What I've gotten to work really well according to HornRESP in scaled imperials.  Note that once you start cutting them in half or other significant down scaling, which reduces the mouth size drastically, they really need to go in the corner for best response:

Eminence Beta 10CX (Adire's HE10.1 kit is a highly regarded set including a better compression driver and everything you need but the wood and is 96db efficient.  I have the 8" version and can vouch for the quality):  This models well in a half width Imperial, but really shines in a 1/2 width and 80%
depth and height.  That approx.  50" tall, 19" wide and 22" deep.  HornRESP shows +/- 1db from 30hz-120hz.

Fostex FE206E -  Highly regarded driver by many.  My pair should be here next week.  Scaling-  1/3 width and 80% depth and height.  Same as above but only 13" wide.  HornRESP shows +/- 3db  33hz-120hz and somewhat smoother response but less extension by reducing chamber volume.

Note that I have not built any of these or any horn for that matter...yet.  I'm going by what the modeling software HornRESP shows me.  One thing I like about the Imperial Horn is that it's a pretty clear line where the horn output stops and the driver direct radiation takes over and I've doubled checked the horn length vs cutoff for proper phasing at the horn cutoff.  

In addition, to being near a corner, the scaled down versions should be inverted so the horn mouth is at the floor.  This is better anyway to get the driver up to ear level and for adding fill to the chamber in the case of the Fostex.

At least for me, the fear factor of building is greatly reduced and I thought I should share what I've found.

The 1/2 imperials Iam constructing have a true horn front and a bit more size near 3/4 what the dual 15in cab .Iam getting near the same depth of bass as the dual but -3db down.Only bad thing about this design is having to attach the horn mids at the side of cabinet.Will post when totaly completed.Thxs for your  post

Carl I saw one of those here in Costa Rica and the price was something over $500 .  If they sound like they are built it would be good.  Keep in mind that PAudio driver is a big heavy PA driver.  I plugged that 12" monster into the half imperial model and it was just as I expected,  102db/1w/1m  +/- 1.5db 32-100hz where the front radiation takes over and still 90db at 20hz.  That's in the corner before room gain.  I just don't know what it will sound like to recommend it.  It will be powerful, but I have a feeling the HF's are going to be harsh.

I have to believe that the Adire HE10.1 kit would make a much more refined speaker you can live with.  The cab would be a foot shorter.  You'd give up a few db of sensitivity and a bit of low end extension, but you'd save $160 plus a good bit on shipping.  Too bad the Adire HE12.1 won't work due to the high Qts, because it would keep up with the Paudio driver.

thanks for your Post John and it mirrors some of your earlier posts on some of the modeling on the Imperial..
I was all they way at the lumber yard picking out wood for the half Imperial a awhile back after your early posts and and changed my mind.
No Imperials for this fella, I am still looking for a place for my Frugals

I believe the original jensen imperials had only one driver per side.

    Brian

Gaz
Its nice to see a post from you, its been awhile. looking forward to the build.
I still have a second bathroom to renovate and still have not got around to the kitchen. But I am not building speakers anymore just hang out and read. I seemed to get more done when I was building speakers because I felt guilty.


gexter

John in CR-
Would a pair of 12" Dayton HO work well in an imperial of any size?  It doesn't meet many of the qualifications you listed.

maybe we should give him a nudge.