If you wanted the perfect loudspeaker it would be what we call a point source. An infinitely small point where a full range of sound comes from. To date achieving this has been impossible.
The penultimate loudspeaker is a single source of sound, one that closely mimics real life where each sound we hear generally comes from a single source: a violin, a human voice, a trumpet. A single driver loudspeaker gets us close to that, but can never be as perfect as the original sound source. That single driver is plagued with a number of problems: FM being the worst.
Imagine a single woofer cone, let's use an 8" woofer for our example. The cone moves back and forth to produce sound. At low frequencies, the cone couples nicely to the air, acting as a good piston, pressurizing and depressurizing the air so we hear sound. If we limit the frequency range to something below 500 times a second, we're in pretty good shape. But when we try and use it as our full range driver we run into trouble. As our imaginary woofer moves back and forth 500 times a second, we are now going to ask it to simultaneously move at 5,000 times a second to get some higher frequencies. What happens? Frequency modulation, sometimes known as Intermodulation (IM for short). It is also known as the Doppler effect, which describes how sound changes frequency with movement of either the source or the listener, relative to each other.
And that's what's happening when our 5,000 times a second tone is moving away from us and towards us 500 times a second. We generate yet another tone, one that is a combination of our two frequencies; an extra tone that's not supposed to be there in the first place.
All this to suggest that while a single driver is great because all the sound comes from just one point in space, like that of a real instrument, there are other reasonably serious problems that most loudspeaker designers avoid by using more than one driver to produce sound.
Tomorrow we'll move on to 2-way loudspeakers in this, a very simplified explanation of speaker types.