Coaxial configurations have some advantages over the more common physically separate woofer and high frequency driver. The most obvious is that the drivers are essentially coincident - that is, they radiate from pretty much the same point in space. Less obvious (but in my opinion more significant) is good radiation pattern uniformity in the crossover region.
I've spent some time with several different coaxials, and have developed an opinion or two. If the configuration calls for a separate compression driver mounted behind and firing through the pole piece of the woofer, the result is a very long throat for the compression driver and this causes audible coloration. In my opinion a better arrangement is for the woofer and compression driver to share the same magnet structure, as this way the throat for the compression driver can be kept very short. Tannoy uses this configuration. I have only found two other commercially available high efficiency coaxial drivers that use this single-magnet configuration. Tannoy doesn't sell their drivers separately, though sometimes older drivers show up on e-bay.
Now there are drawbacks to the coaxial configuration. For one thing, a very important part of any round horn is the mouth. It should be smooth and have a large round-over or "lip" radius. The perimeter of the woofer cone is the mouth of the horn for a coaxial compression driver, and it is far from ideal. The result will be on-axis response anomalies. From 10 degrees or so off axis, the response will be much smoother.
That being said, I'm presently working on two designs that use coaxial drivers - so I think the configuration has promise.
Duke
