Doppler flowmeters exploit the Doppler effect, which is the shifting of frequency resulting from
waves emitted by or reflected by a moving object. A common realization of the Doppler effect is the
perceived shift in frequency of a horn’s report from a moving vehicle: as the vehicle approaches the
listener, the pitch of the horn seems higher than normal; when the vehicle passes the listener and
begins to move away, the horn’s pitch appears to suddenly “shift down” to a lower frequency.