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The best midrange comes from compression drivers, it has that minuteness, transparency and dynamics that no other topology can offer; the main problem with them is that they only cover a very specific mid frequency range.  Their sound is so distinctive that it is very hard to mix it with other speaker topologies.  They have been mistakenly used above a 15” driver with the terrible results by which we judge horns badly.

The only way to fill in the frequencies below the compression driver is with a bigger horn; the Upper Bass Horn.

Compression drivers work well from around 15 khz on the upper range to 800 hz on the lower rage.  Most are specified for 500 hz and recommended at 1,200 hz.  So let’s use this range as our upper range target.  About how low they should go, well; as low as physically possible!

Bruce Edgar designed his first Upper Bass Horn to go down to 80 hz, it was a square horn with a 12” cone driver.  I am very familiar with the sound of the latest more conical version.   It does have some horn gain but it gets down to 80 hz with almost no compression and dead sound.  Bert Doppenberg made a nice 150 hz Oris horn for Lowther drivers,  but with no compression,  the horn works mainly as an equalizer, I heard them a lot of times and built a few,  lowther drivers can cover a very long range inside a horn but their sound becomes very soft,  very listenable; but no match for compression drivers.  Adding compression to a Lowther driver will spike the sound but they will hardly go below 200 hz, and since we only need them up to 500 hz, they become redundant,  there are more powerful pro drivers that will cover lower frequencies with much more elegance and transparency than Lowthers will.

Of course the best thing to do is a Round horn that will cover as much range as logically possible.  80 hz will need a mouth size of ¼ of its wave length: That is about 110 cm, now the ear of a human being sitting down on a comfortable position is at 90 cm.  For good soundstage the compression driver playing the higher frequencies must be at ear level.  The normal horn size for a compression driver with a tactrix profile is around 28 cm.  If we smack it in the middle of our ear it will leave 76 cm under it, this is the maximum size our Upper bass Horn can have in order to allow the compression driver to play at ear level.  A mouth of 80 cm at ¼ wave length is about 110- 120 hz. My UBH is 82 cm high.

Now in order to make a cone driver work inside a horn and match perfectly with a  compression driver it should work with compression in order to have a sensibility gain from 6 to 9 db, making a readily available 8”, 10” or 12” driver with a sensitivity of 100 db go up to 108 db.  It is not so much as to match level, but manly to match output, speed and dynamics.  The properties for this driver may vary, but certainly you will need an FS close to the horn mouths cut off,  high sensibility, very light cone for faster transients, etc.

This all sounds very easy and logical; so you have to make your round horn of 76 cm in diameter with a cone driver inside it, add some compression, basically choking it with a throat around 50% smaller than its cone diameter, and hope it will get up to 500 hz and as low as we can get away without honk.  The driver you choose will have to go inside a “box” otherwise there will not be any pressure build up.  The size and volume of this box has to be specifically calculated for throat reactance, whatever the size will be, adjustments will have to be done when changing to another driver, so the best thing to do is leave it “movable”, either you can make it big in order to open it and add something inside it that will take up volume, or you can make it adjustable with a sliding wall.  I made the last and let me tell you ¼ of the turn of a bolt will make a big difference in sound.

Of course making the horn is no easy task,  and I wont even go there. Let’s say you already have the horn, picking the right driver and getting to the perfect back chamber volume will take you a long time and extended listening tests, I have literally chased a resonance on the horn from its lower range to its higher range, only to bring it back down again and eventually out of the horns cut off in order to get rid of it.  Once it is almost out of your horn; if you turn the back chamber volume bolt half a turn more the sound of your horn will die, put it back closer and it will just jump out of the horn with the most transparent and clean sound you ever listened this frequency at.

I have made several UBH, from 180 hz down to 120 hz, with exponential and Tactrix profiles.  The 140 hz horn will do most things nice but the cut off is still at a very sensible frequency.  140 hz will most probably bring you in room anywhere from 160 hz to 130 hz depending on the position in your room.  Once you get the sensibility to hear the resonances of the horn down low and where it dies out, you will notice this is a very bad range to let it die, a lot of fundamental frequencies of the voice and several instruments are still going strong, so a performance “step” will appear between your UBH and the channel below it, mainly if it is the same 15 inch woofer!  These were very time consuming an expensive tests to fail, 120 hz will get you just out of that range and still into the range a woofer that can go under 30 hz will cover.   Make it 140 hz and you will need two separate bass solutions, one that goes from 140 hz down to 40 hz and a subwoofer.

You want aggravations; let your woofers start a fight between them, when you can get a higher bass output by lowering the volume level on one woofer amp,  something is not right!

Once your UBH is running you have to listen to it by itself and make sure all the resonances are out and you still have the great dynamic sound.  Now get your RTA out and measure what you got.  Be sure to place a high pass xover below the natural cutoff of the horn, if you leave lower frequencies to be played by the horn they will mess up the sound, so unload the extra bass out of your horn. You will most probably be able to mix in your compression driver very naturally; the point where the UBH will be dying on the upper range will most probably be well within the range of your compression driver.  You can play around with low pass cuts on your UBH and high pass cuts on your CD in order to get the tone and timbre you like, make it sound a little fuller with more overlap or cleaner and drier with more separation between the drivers:  since both drivers are at the end of their natural frequency ranges you will be able to mold their sound properly with a first order xover.

If the sensibility of your UBH does not match the compression drivers one perfectly, you will need to use a separate amplifier for each horn, resistors at speaker level or Lpads here will kill a lot of harmonics and tone.  IF you need more sensibility, you can add compression to the horn or change the driver inside it.  Some drivers won’t be able to get a nice full tone and explosive dynamics once inside the horn with compression, lowther drivers don’t do very well, Fostex wont either, but even some drivers that develop nicely inside a horn can give way to a driver that will work wonderfully inside a horn.

In order to change the tone of both drivers, horn size changing is a very strong tool, drivers will most likely do what the horn demands of them, but they will pay little attention to crossover changes.

Mixing in the two horns, the UBH and the Compression driver is a lot of fun,  if you never listened to an UBH and CD playing together you will be so impressed,  you will probably just leave it on the most comfortable position and enjoy the sound as is!  Don’t, time alignments will bring the sound even closer together, and careful volume match will be the easier way to make those wonderful microscopic tonal adjustments that will make your system explode with color!

Jorge

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