The Doppler effect, named after a German physicist (how come things are always named after a German physicist?), is the apparent change in pitch of the sound that occurs when the source of the sound is moving relative to the listener. For example: A car horn will sound higher in pitch as it approaches, and lower in pitch after it passes us. This is one principle that is employed in a rotating speaker system like a Leslie. The rapid movement of the horn to and away from the listener creates a sort of vibrato effect. There are many modern effects units that simulate the Leslie sound, and also offer other types of Doppler effects.
If a loudspeaker is producing both low and high frequencies, the low frequencies will cause the cone to move alternatingly toward and away from the listener (obviously high frequencies do this too, but the lows are much more pronounced). As this is happening the perceived pitch of the higher frequency sounds rise and fall at a rate (or rates) equal to the low frequencies moving the cone. This is actually Frequency Modulation of the high frequency by the low frequency, and is called “Doppler Distortion.” It manifests itself as a sort of “muddiness” (subjective audio term #108) of the sound.