While any driver can be horn-loaded, when people talk about horns, they usually mean a compression driver with a horn attached to the front, either through bolting (the standard for audiophile equipment) or via a threaded, screw-on junction (the standard for professional equipment). Relative to normal drivers, compression drivers have very low distortion due to the small size of the membrane and the relatively short distance the membrane needs to travel.
So, as I began looking into mid-horns, most of the information available online suggests that you will need to purchase vintage equipment, such as 1950s-era JBL compression drivers and similarly aged JBL or Altec Lansing horns or else build your own horns, which can be a daunting task to say the least. If you go on eBay and look for these products, you'll see that very little is available for less than $1,000, as they have reached the status of collectors' items and are priced as such.
However, I also noticed that horns and compression drivers are still used quite extensively in professional audio settings, such as stage monitors and PA speakers. In fact, Selenium's D250-X compression driver has performance specs that looked very similar to those of the multi-thousand-dollar, vintage compression drivers I was seeing on eBay, including an uncrossed frequency response from 400 Hz to approximately 9 kHz and a sensitivity of 107 db at 1w/1m, but with a power handling capability of 150w (vs. very low power handling on vintage equipment). The biggest difference, though, is the price: ~$35!
Likewise, when looking into mid-horns, the prices for newly constructed or high-quality vintage equipment usually range from ~$750+ up to tens of thousands of dollars. This is simply too expensive for me, so I began looking for other options, including DIY solutions and professional audio. At this point, I had to make some decisions, including which horn curve I wanted to use.
Deciding on horn curves is basically a matter of weighing a series of compromises and figuring out what matters to you most/least. While conical horns tend to have the least honky horn coloring and are also easy to construct, owing to their straight-angled sides, they are also the least space-friendly and also tend to be very ugly, in my opinion. Exponential horns take up considerably less space than conical horns for a given frequency response, but they are more difficult to construct, due to their curved sides. Similarly, square/rectangular horns tend to have better dispersion and listening angles than round horns, while round horns impart less distortion to the sound wave.
I decided that a round exponential, while difficult to construct, would be my best bet, since I want to experience the horn sound and my space is limited (my listening room is less than 200 sq feet).
While weighing my options, I came across some potentially good options in the pro realm, including the Dayton Audio H12RW 12" waveguide and the Selenium HL14-25 exponential horn. The waveguide was attractive to me because it can easily drop right into an existing speaker cabinet designed to house a 12" woofer. However, waveguides are a little different from traditional horns, and that's really what I wanted to try. That in mind, I went with the HL14-25 horns, which have a suggested frequency range from approximately 600 Hz to 5 kHz, well inside the range of the driver.
When I received the horns, I first tested them by wiring them in series with my Phase Technology Teatro 7.5s (8 ohm, 90 db sensitivity, freq. resp. of 40 Hz to 20 kHz), but the sensitivity of the horns was too far off from that of the Teatros, such that the horns were ear-splittingly loud by the time the volume was high enough to hear the Teatros comfortably. With that in mind, I resurrected my old Sony solid state receiver (100w per channel) and wired it to push the Teatros and my subwoofer while my Dared VP-20 pushed the Selenium horns. This way, I could independently adjust the volumes to achieve a good balance between the horns and the Teatros, and then use my preamp's volume control as a sort of 'master volume.'
To run my initial tests, I left the horns uncrossed and expected a good bit of distortion below the 400 Hz mark, since that's as low as the drivers are supposed to go. I cautiously kept the volume very low as I didn't wish to blow my new drivers on their first go 'round. However, I was pleasantly surprised to find that the horns handled the lower frequencies quite gracefully, with no distortion that I could notice--even when playing as loud as I could comfortably stand--and with a fairly smooth (though rapid) rolloff below around 600 Hz. I'm assuming this success is largely due to the fact that the horns are probably receiving only a handful of watts instead of the 100+ watts that are normally shoved through them in professional applications.
Based on this experience, I have decided not to bother with a crossover at this time, though I may revisit the issue in the future.
I guess the real question now is: "How does it sound?" In short, it sounds fantastic. The frequency range covered by the horns encompasses guitar solos, most vocals and organs, which all sounded a bit anemic on my Teatros alone. The clarity and definition in these frequencies is truly astounding and must be experienced. If you've only ever heard cone drivers for this range (like me before I bought these horns), it's honestly impossible to imagine how rich the sound reproduction can truly be.
My setup is designed to allow for extensive A/B testing (in addition to the independent volume control for the Teatros and horns, the VP-20 has independent volume control for each channel), and this is where the improvements really shine. Even my wife (who is very honest about differences she can and can't hear, often to my chagrin) was blown away by the disparity between Teatro+horn vs. Teatro alone.
The horns admittedly have some strong directivity, but they also have a good--though different--sound off-axis, such that listening outside of the sweet spot is still nice, just not the same as getting right there in the middle.
Here are a couple of pictures of the horns sitting on top of my Teatros:
In the first picture, you can see the VP-20 that pushes them, as well a little bit of my disc changer, preamp and crummy solid state amplifier.