I'm going to change things up a bit and focus a little more on the drivers that I chose for this project. I know many people only use elements of these designs, or use one driver and change another, so I figured that some more specific info about the drivers would be helpful.

Hammer 18CLF: This surplus driver is a limited-supply item, but is nearly identical to the new Eminence Definimax 4018LF 18" driver, so my description should hold true for both. They are very powerful-looking 18" woofers, and feature a massive cast aluminum frame, high-quality coated paper cone, nicely vented motor structure, and copper shorting rings for reduced distortion.

Frequency-wise, this woofer has a smooth response, without significant breakup peaks or other anomalies. It has a strong response up through 200 or 300 Hz, where it begins to diminish in output. Thanks to the shorting ring in the motor structure, the bass and midbass is very clean and tight.

Based on my experiments and use of this driver, I would say that it would be at its best when used in smaller vented cabinets with slightly higher tuning frequencies than what I used in this design. With my alignment, the low-end extension was impressive for a PA speaker, but was missing a bit of the "snap" and "punch" normally associated with output around 50 or 60 Hz. With a smaller box, higher tuning alignment, four or eight of these running from 40 -120 Hz would produce some killer bass output. If you're looking to add some "bass bins" to your PA rig or club, these would be an excellent choice.

Hammer 8SLF: Once again, this surplus driver is of a limited supply, but is interchangeable with the Eminence Beta-8A for all intents and purposes. In my opinion, the Beta-8 is one of the best all-around high-sensitivity 8" midbass/midrange drivers available, and is a steal at its price point. While it doesn't appear to be anything special in terms of construction, it has the perfect combination of cone, surround, spider, and dustcap to provide very smooth and clear midrange without any harshness or breakup peaks.

It is an open-backed driver, which means that it does require a rear chamber if it is sharing an enclosure with a woofer. This gives it the flexibility to be put in small sealed enclosure if it is being used as a dedicated mid, but it can also be used in larger vented enclosures as more of a midbass driver. When using it in a small sealed enclosure, it is critical to properly damp the rear chamber to prevent rear-wave reflections from distorting the sound. In this design, I was able to clearly see these reflections in my preliminary measurements, and had to make sure I fully damped the rear chamber with a combination of foam, fiberglass, and Acoustastuf.

All-in-all, this is a very versatile 8" driver, and would be a great choice for any high-sensitivity application. I've even used it from 80 Hz to 2.5 kHz in a home theater surround-sound system, where the high sensitivity provided an excellent theater-like experience even with a modestly-powered receiver.

Eminence ASD1001S w/ Dayton H07E Waveguide: The Eminence ASD1001S is a relative newcomer in pro sound high frequency drivers, and features an unobtrusive appearance as well as price. For a mere $30 you get a driver with a titanium diaphragm, nice quality spring terminals, and high sensitivity.

On paper, the frequency response of this driver is not the greatest, showing a rolloff between 5 kHz and 10 kHz, with some extra zing up around 18 kHz. My measurements coincided well with the Eminence supplied measurements, which is nice to see. Eminence doesn't publish this data, but it is in the distortion domain where this driver really shines. Distortion is extremely low down to around 2 kHz, and remains low throughout the entire range. The high-frequency rolloff is actually a result of the driver staying linear (where most similar drivers are full of breakup peaks and ringing); this combined with the overall low distortion makes for a very non-sibilant, easy-to-listen-to driver.

Since the ASD1001S is a driver only, it does need to be mated with a lens of some sort for implementation into a design. I tested many waveguides and horns as part of the design process before settling on the Dayton H07E. This waveguide provided minimal disturbance of the natural response of the driver, while providing good loading down to 2 kHz. Smaller horns could not provide this low-end loading, while the larger horns tended to disrupt the high-frequency response. This waveguide seems to be an excellent choice for applications where high-fidelity is paramount over high-sensitivity.