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In the classic textbook example, the Doppler effect is demonstrated by an increase in both pitch and volume (or amplitude) of a train's whistle as it approaches a station, followed by a decrease in pitch and volume as it moves away. This effect---the shift of a frequency emitted by a moving object---leads to a fundamental flaw in audio technology. A midrange driver behaves like the approaching-and-departing train when it attempts to reproduce varying frequencies. When the driver is fed simultaneous 400Hz and 2kHz tones, the forward movement of the cone at the lower rate modulates the 2kHz tone upward in pitch and amplitude; when it moves backward it modulates the higher tone downward. (The human eardrum also behaves this way, but the brain's audio-analysis circuitry knows how to deal with it.)