RobbieZ
I was wondering, most CAB’s use 12” speakers. Why not 10 or 8”?
Chabenda
Robbie - I think that, to put it quite simply, the 12" speaker has the best balance of treble, mids, and bass. 10's and 8's aren't generally capable of supplying the mids and bass that we require.
GuitarDoge
An 8" or 10" is good enough for low volumes, but as soon as you turn it up louder the bass they produce doesn't keep up with the treble, making it sound a little lacking.
It's the same reason why subwoofers have the biggest speakers in a home surround system.
Scientifically it's: bigger cone = bigger wavelength = lower frequency = more bass
AlanRatcliffe
In short, player preference. 12's for live play and 10" for recording. Especially in a combo format.
It's difficult to generalise with speakers as specific speaker and cabinet designs make a huge difference, but the popularity of 12s for guitar is likely due to the generally extended, softer low end and wider throw of a 12".
Multiples change everything - a 2 x 10 has more surface area than a 12" (almost the same as a 15"), so (generally, once again) can go as low (or lower), but has a tighter, more defined character. That's why modern bassists like their 4 x 10s (and why we of the old fart bass brigade like 1 x 15s - for their fatness and smoothness).
Economics play a role too - While a 2 x 10 array will fit most amps made for 12", they generally work out more expensive.
Wildaker
A lot of it's tradition, too. Go back 50 or 60 years and 12-inch speakers were, generally, the most useful general-purpose guitar speaker, in an environment where the back line also provided the front-of-house sound. Now that everything is mic'd up, there's nothing to stop you playing a huge open-air festival with a 1x10 (and I know a man that did just that), but there's something iconic, for certain types of music, about standing in front of a 4x12 or four, and people keep on doing it.
Pete Townshend, the guy who first popularised the 4x12 stack, now plays Fender Vibro-Kings with 3x10-inch speakers (backed up, admittedly with 2x12 cabs).
Me, I like 10s, and I'm thinking of having a 2x10 baffle made for my Deluxe Reverb...
DaFiz
A couple of years ago in a weekend band I played bass guitar through a borrowed Ashdown amp with 300W into 4x10" speakers which produced a better tone than my own Ashdown 300W into 1x15" speaker... Depends on what you're after I supose ?
RobbieZ
Thanks for all the answers. I will do a bit more research before making my final desition.
