Andy, it'd be easy to write a book about this, and I'm 2-finger typer on a damned French keyboard on vacation in Grenoble, so I gotta keep it short...plus I'm not the expert here.
Driver diameter is chosen to balance many needs. Generally, the larger the driver (and motor), the lower the passband, the narrower the dispersion at higher frequencies.
Simply, it gets hard to mate an 8" driver with a 1" tweeter, as there passbands barely intersevt smoothly, and off-axis response usually suffers from excessive "flare".
It follows that the necessary low crossover for such a design risks overpowering the more fragile tweeter's motor.
Rare is the 8+1 that sounds great (I do remember a Genelec pro monitor that was pretty impressive, but again probably intended as a console on-axis monitor rather than normal room use.), but it's a cheap way to get a lot of bass, generally as a trade-off with smooth midrange.
Smaller mids have smoother upper freq response, so usually will result in better blends with tweeters, sometimes allowing the crossover freq to be pushed WAY up, thereby reducing all sorts of crossover response crap in the critical midband. (B&W crosses the Nautili at 4k, but not to my personal liking; Verity Audio uses such a fine mid that they don't cross it until way up at 5500Hz! The "coherence" is phenomenal, as a result...but I digress.)
Since small mids don't work too efficiently down low a designer has to decide whether to boost the bottom with a vented or very large enclosure, or of course simply design the speaker with limited bass. The former may work very well within its dynamic range, the latter may benefit from boundary support or of course a subwoofer. The obvious rejoinder is to add a third driver for bass response, taking the load off the mid, and thus allowing a smaller mid that blends better with the tweeter.
In practice one sees a lot of 5" and 6.5"ish +1" monitors. GENERALLY the larger ones are punchier, having greater dynamic capability, and a more open sound. Many 5" mids sound pinched to me, with a "cupped" coloration. In my own work with an 8+5+0.75 three way I couldn't quite get the lower mids to sound as natural as with my 6.5+1, for example. Of course many designers DO have tremendously successful designs with 4-5" mids, as the physics postulates that size range as most appropriate for the female voice, for example.
Designing ANY three-way design gets much trickier in practice, despite the easy passband summations of 8-10" + 4-6" + 1"...but this is a big subject.
You noticed an inverse correlation between midrange size and price. This is a personal idiosyncracy, and perhaps the result of comparing a well-honed smaller design using a great midrange driver vs a larger design using a cheaper "mid-bass" driver. I dunno. The popularity of 6-7" + 1" two-ways indicates the success of balancing fine, well-balanced response and cost. Since it's a bit easier to improve upon the critical midband with a smaller driver, a designer will (almost) necessarily give up bass response to achieve this holy grail...sometimes the results command a higher price, even though the driver is smaller. There are too many other factors involved, some of which I hope I shed a bit of light on. A bientot. Ern
Driver diameter is chosen to balance many needs. Generally, the larger the driver (and motor), the lower the passband, the narrower the dispersion at higher frequencies.
Simply, it gets hard to mate an 8" driver with a 1" tweeter, as there passbands barely intersevt smoothly, and off-axis response usually suffers from excessive "flare".
It follows that the necessary low crossover for such a design risks overpowering the more fragile tweeter's motor.
Rare is the 8+1 that sounds great (I do remember a Genelec pro monitor that was pretty impressive, but again probably intended as a console on-axis monitor rather than normal room use.), but it's a cheap way to get a lot of bass, generally as a trade-off with smooth midrange.
Smaller mids have smoother upper freq response, so usually will result in better blends with tweeters, sometimes allowing the crossover freq to be pushed WAY up, thereby reducing all sorts of crossover response crap in the critical midband. (B&W crosses the Nautili at 4k, but not to my personal liking; Verity Audio uses such a fine mid that they don't cross it until way up at 5500Hz! The "coherence" is phenomenal, as a result...but I digress.)
Since small mids don't work too efficiently down low a designer has to decide whether to boost the bottom with a vented or very large enclosure, or of course simply design the speaker with limited bass. The former may work very well within its dynamic range, the latter may benefit from boundary support or of course a subwoofer. The obvious rejoinder is to add a third driver for bass response, taking the load off the mid, and thus allowing a smaller mid that blends better with the tweeter.
In practice one sees a lot of 5" and 6.5"ish +1" monitors. GENERALLY the larger ones are punchier, having greater dynamic capability, and a more open sound. Many 5" mids sound pinched to me, with a "cupped" coloration. In my own work with an 8+5+0.75 three way I couldn't quite get the lower mids to sound as natural as with my 6.5+1, for example. Of course many designers DO have tremendously successful designs with 4-5" mids, as the physics postulates that size range as most appropriate for the female voice, for example.
Designing ANY three-way design gets much trickier in practice, despite the easy passband summations of 8-10" + 4-6" + 1"...but this is a big subject.
You noticed an inverse correlation between midrange size and price. This is a personal idiosyncracy, and perhaps the result of comparing a well-honed smaller design using a great midrange driver vs a larger design using a cheaper "mid-bass" driver. I dunno. The popularity of 6-7" + 1" two-ways indicates the success of balancing fine, well-balanced response and cost. Since it's a bit easier to improve upon the critical midband with a smaller driver, a designer will (almost) necessarily give up bass response to achieve this holy grail...sometimes the results command a higher price, even though the driver is smaller. There are too many other factors involved, some of which I hope I shed a bit of light on. A bientot. Ern