The term given to the apparent change in frequency of sound or light waves depending on whether it is moving toward or away from the listener (such as the change in tone of emergency vehicles or cars as they pass by). This effect is also noticeable in electromagnetic waves, and is used in ultrasound scanning to listen to the fetal heartbeat.
Effect responsible for train whistles sounding shrill when approaching and low pitched when receding. When a moving platform emits sound waves, the waves are compressed in the direction of platform velocity and rarefied (spread out) in the opposite direction of velocity. The compression of the waves raises their frequency, yielding a higher note. This also applies to a ping or pulse reflected off a moving platform – the return pulse will be up-shifted or down-shifted depending on the contact's motion toward or away from the hydrophone.
The change in frequency of an emitted wave when it encounters a particle moving relative to the transmission site.