Doppler effect with moving structures

Pairs of transmitting (T) and receiving (R) transducers illustrate the Doppler effect as it occurs in relationship to targets that are stationary, moving toward, or moving away from the transducer. With a stationary target (panel A), the carrier frequency [f(t)] from the transmitting transducer strikes the target and is reflected back to the receiving transducer at the reflected frequency [f(r)], which is unaltered. An increase in f(r) is seen with a target moving toward the transducer (panel B), while f(r) is reduced with a target moving away from the transducer (panel C). In all cases, the extent to which f(t) is increased or reduced is proportional to the velocity of the target.