kingharold is correct. Music is in octaves. If one considers the 3.5 kHz crossover, in a two-way system, the woofer then covers 7-1/2 octaves! If there is a midrange speaker crossing over from a woofer at say 250 Hz, the woofer then covers about 4 octaves, while the midrange covers about 3.5 octaves before crossing over at 3.5 kHz.
As audiokinesis posts, the ear is most sensitive over 700 to 7000Hz range, with the most sensitive in the ~1-3 kHz zone. It then makes sense to have a very good midrange speaker crossed over below 500 Hz and maybe above 8 kHz, ballpark frequencies, so that the most sensitive hearing range has no crossovers at all, so no large phase transitions over that range.
This is a design problem with compromises galore.
As audiokinesis posts, the ear is most sensitive over 700 to 7000Hz range, with the most sensitive in the ~1-3 kHz zone. It then makes sense to have a very good midrange speaker crossed over below 500 Hz and maybe above 8 kHz, ballpark frequencies, so that the most sensitive hearing range has no crossovers at all, so no large phase transitions over that range.
This is a design problem with compromises galore.