In the old days, it was fairly typical to use a 10-inch or 12-inch driver for the bass in order to have enough cone area to move enough air to produce a lot of bass.
This resulted in a fairly wide speaker enclosure, usually 16 inches wide or more.
With the rise of home theater in the last 15-20 years, people often want speakers that are less than 9 inches wide to fit on either side of a large TV monitor ..."tower" speakers.
The solution is to substitute multiple drivers with two or 3 smaller speaker cones, often 6.5 inches, wired in parallel, to give the same cone area and air motion/bass as one larger-diameter driver. An advantage of this is that the smaller drivers have lighter cones and can have a greater excursion without distortion.
The typical configuration is to have 2 or 3 bass drivers, a smaller midrange driver of 4 inches or so, and a small tweeter, creating a 3-way system.
In my Gallo Acoustics CL-3 speakers, two ultra-lightweight long-throw carbon-fiber 5-inch drivers with transmission-line loading team up to go from 32 Hz to 3 Khz where they roll off naturally and then a tweeter takes over from 3 Khz up, completely eliminating the need for an electrical crossover ( or two) with the usual phase shift problems.
Edited by commsysman - 7/8/12 at 11:24am