I don't think you can say that any one design is superior to the other - as I mentioned earlier, it is down to how well the design is implemented and executed. In theory, a single full range driver should be the best in this respect, for obvious reasons. But, you'll usually find that these types of drivers aren't as good at the frequency extremes as it is in the upper bass to lower treble. This is where a two way comes in, so you have a driver that can deal with mid and bass, and let a separate driver do the higher stuff. This allows the drivers to do less work, and reach farther out into the frequency extremes. The drawback with the two way is the crossover, which can have a detrimental effect depending on how well it is designed. Most speakers will be two way, because its cheaper and easier to produce. A three way speaker allows the three sections of the frequency range (bass/midrange/treble) to be concentrated on by a driver that is specifically designed for the job. In a two way the bass driver has to reach as low as possible as well as reproducing midrange, which tends to affect the quality of the latter. A three way system means the bass tends to be cleaner because the driver doesnt need to produce frequencies above those that it is comfortable with, and it wont negatively affect them either. The midrange gets its own driver, so doesn't have to compromise itself by using a large roll surround, or directly affect frequencies well above what it really needs to do (or you could say, what it shouldn't be doing). It can sound cleaner because it isn't negatively affected by large cone movements. The treble unit can play cleaner and more precisely because it doesn't have to go deeper than it is comfortable with, because that load has been eased by the midrange driver. All in all, everything sounds cleaner and tighter because of the dedicated drivers.
Other speakers may steer clear of the three way design and utilise two mid/bass drivers, which increases efficiency. Using two drivers shares the work of one, so the bass cones don't have to travel as far to reproduce low notes. As well as higher power handling, this can mean a cleaner midrange, and a tighter bass because of the lower cone excursion. The drawback with timing probably isn't too much of an issue as they're both on the same (front) face of the cabinet, and will be the same distance from you, so won't cause timing issues. Those two bass drivers, perhaps working up to 3kHz as an example, will be producing a lot of directional frequencies (above 150/200Hz as a rough guide), and with two drivers producing the same frequency range, you're creating a perceived 'centre' for those frequencies, which is in between the two mid/bass drivers. This is shifting the focus of those frequencies further away from the centre of the treble unit - unless you place one mid/bass driver above the treble unit and one below - you now have a perceived centre which is the point of the treble unit. This creates, very basically, the illusion that a coincident driver produces - all frequencies coming from a single point. There are still issues with this compared to coincident, but that's for the designer to worry about, and I'm digging myself deeper here 🙂
So ideally, all the drivers should be placed as close to each other as possible in order to try and produce a coherent sound that isn't 'smeared' due to multiple drivers here there and everywhere. An example of how to do something like this well is the KEF Blade - treble and mid down to 350Hz on the front face of the speaker (UniQ array), and the rest produces by four bass drivers, two on each side of the cabinet. These bass drivers are quite a distance from the UniQ array, but placed in such a way that from the listening position it sounds like all frequencies are coming from the same point. From a timing point of view, you'd expect the whole sound to be quite askew, but all this has been taken into consideration during the design - after all, there'd be no point producing a speaker that is 'reference' in some areas and downright awful in others.
Jack from KEF will be able to explain it all better than me as he is a speaker designer (and I'm not), so I'll see if he can jump on and give his take, or correct anywhere I've gone wrong.
Just one thing to add that I've just thought of regarding dual mid/bass drivers - I've always felt from experience that a larger, long throw single mid/bass driver (like those we used to see in bigger speakers in the 70's) produced a nice, weighty, full sound. In comparison, two smaller short throw drivers seem to lack that fullness, but can be noticeably faster - some may say leaner too, but I'd actually say more accurate, and able to pick out subtleties that can be lost in a larger driver. So overall, I'd prefer a few smaller bass drivers to one larger driver. I'd say its the same with subs - I'd rather have two smaller drivers than a larger single driver (with equivalent area). The larger driver can sound initially impressive, but won't sound as fast or be able to produce tiny subtleties like the two smaller ones.
Anyway, I'm off to watch a film before its too late!!
Facebook
Twitter
Instagram