First listen to just the loudspeaker on top (fullrange, without the active crossover in other words). It takes some time to get used to mono, so play about 30 minutes worth. Next introduce the active crossover and feed the highpass signal to the top loudspeaker. Now listen to the same 30 minutes of music.

    Loudspeaker diaphragms move in response to signals and the large diaphragm movements caused by low frequencies raise the high frequencies while the driver's cone moves toward us and then lower the high frequencies when the cone moves away from us. Consequently, in absolute terms, it is impossible for anyone driver to reproduce more than one pure tone at a time. This is one reason why electrostatic and planer speakers sound as good as they do; the large surface areas do not need to move very far to produce high sound levels. Three-way, four-way, five-way, and six-way loudspeaker also greatly reduce this distortion, while admittedly adding huge crossover problems. And the worst offenders are small fullrange single driver based loudspeakers.

Bi-Ampped Identical Loudspeakers
   I once heard something like the test setup with four large Advent loudspeaker from the early 80's. A friend bought four speakers for quadraphonic use and instead used all four in a stereophonic setup with a active crossover set at 100 Hz. The improvement in midrange clarity and bass definition was markedly better. In fact, it sounded much better than it should seem possible. (A similar experience occurred when I heard a speaker setup that consisted of eight high quality mini-monitors configured as two speakers. No crossover was used and four speaker enclosures were stacked on each other per side, with only one facing the listener. The resulting impedance was identical to that of a single speaker, as they were wired in series-parallel. Because each speaker saw half of the available output voltage from the power amplifier, its response was down 6 dB; but as the radiating area had increased fourfold, the output response gained +12 dB of gain, which in the end, yielded +6 dB of gain.)

    If the crossover is working well, the sound should be very similar to that from the single loudspeaker. The usual result is that the sound is much better than that from the sole speaker. Why? The power increasing effect is part of the reason. The rest of the answer is found in the reduction in distortion that results from freeing the top loudspeaker from the onerous task of reproducing low frequencies. In other words,  the top loudspeaker is less likely to mechanically clip. Furthermore, there is a large reduction in a seldom mentioned distortion: frequency modulation distortion, FMD.

FMD
    This distortion is related to Doppler distortion: the phenomenon of frequencies increasing in pitch as a sound producing object nears us and the sound decreasing in pitch as the object retreats. The sound of a race car approaching and passing is the usually given example. RRRRR R R R  R  R  R   R   R   R   R.

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