Originally Posted by notnyt
The waveguide is not going to make up the differences that I heard. Sorry.
this one is actually a little tricky. the m2 waveguide is broader (120 deg vs 90 deg) and a design goal was to optimize the consistency of the response as one moves off axis in a little bit different way than constant directivity.
this picture tries to show what is going on (not too successfully to my eye).
what it is trying to show is that the actual response of the waveguide is quite similar not just in shape but in output level across a wide angle. in some ideal theory, the radiation would be uniform in power in all directions.
with a constant directivity horn, such as the 2384 in the 4722, the high frequencies (the blue and the purple) remain roughly the same flattish shape, but lower in output as one moves off axis. that is great. easy on the brain, but identified as reflections, not part of original sound. the strawman speaker on the left shows the blue dips significantly off axis while the purple remains high. that is just a speaker without constant directivity to begin with.
anyway, with a near constant *output* in all directions, the sweet spot is broadened and the imaging is enhanced. in a reverberant room where the listener's brain is trying to integrate all of the reflections along with the direct sound, when the reflections are all of the same shape and amplitude, the brain doesn't try to "reject" some of them while "accepting" others--they all make sense and so all contribute to the image effect.
obviously the larger waveguide in the m2 (compared with the 708 or even the little guys) holds this effect to a lower frequency. just about everybody who owns the m2's or who has heard them in a reverberant room has commented on their ability to image a big soundstage that expands out past the speakers as well as in between them. anecdotal I know, but indicative of the theory actually working.
the measurements that you showed where the larger cd's beat out the m2 cd (d2430k) for low end extension, distortion, and your comments on subjective sound quality are solid (and much appreciated btw), but hitting on different aspects of overall performance than what I suspect Gooddoc was referring to in his comment about the m2 horn.
as for the comment about "refinement", it might be worth noting that adding the second woofer increases output by about 6db. viewing the second woofer as merely an "output" device though may miss some of the benefits. since distortion scales roughly at twice output (i.e. each time output increases by about 3db, thd will increase by about 6db, very roughly speaking), this means that the second woofer could also be viewed as a "refinement" device where for any given spl level, the distortion is reduced by about 12db.
the m2 is designed to be usable almost nearfield though, so a full range second woofer is not practical or desirable.
in the sweet spot in a non-reverberant room, smooth and consistent frequency response with low distortion and minimal power compression is the ante and they both bring that to the table (ironically, the largish open rooms/auditoria where the m2's are frequently demoed may not actually reveal one of their primary advantages). damping the ti phragm (or going whole hog on beryllium) takes the 4722 to the next level and appears to put the two on at least equal footing there or even giving the advantage to the 4722. in a semi-nearfield and/or reverberant room, the m2 with its more uniform power response may provide a better experience. maybe each one brings a little something different to the show and depending on what the application/environment/constraints/budget is one may be a little better than the other.