Thank you for this, just excellent. let's take this one more step : do u think that a single driver speaker, with an equalizer (inboard or outboard) could possibly be high - fidelity? Or is this simply not practical?
I am not a speaker designer, but I do not believe it practical, though different folk have different definitions of "high fidelity". IMO a single driver cannot solve the physical issues of beaming and need for high excursion with extremely rigid cone surface to provide adequate output and sensitivity over the full frequency range. And as
@SSS mentioned you have the Doppler, or as I learned it frequency-modulation distortion, that occurs when a single cone is required to reproduce such a wide range of frequencies. You'd need to approach a point source design with very small radiating surface and very long throw (excursion) and the physics are against you. I'll go out on a limb and say it is impractical to get sufficient LF output with low distortion from a very small radiator, and impractical to obtain broad dispersion with low distortion from a large driver suitable for bass. The closest practical single drivers, as mentioned earlier, are large panels that are segmented to improve their dispersion, and even then it is very difficult to obtain high LF SPL with low distortion as it is difficult to provide a linear voltage (ESL) or magnetic (planar-dynamic) field over a long enough range (excursion) and/or large enough panel to produce the required LF output.
Multiway speakers are used for good reason, though getting smooth response across the midrange is a challenge because it spans so many octaves. The choice of drivers and crossover design become very critical to provide a smooth response across the entire "midrange" region of say 300 Hz to 3-8 kHz or so. But they solve more problems than they create, at least most of the time for decent designs. I have to think single-driver speakers would be cheaper and easier to manufacture so if they were a viable solution we'd see more of them.
IME/IMO - Don