Answer / guezz who

is the change in frequency of a wave (or other periodic
event) for an observer moving relative to its source. It is
commonly heard when a vehicle sounding a siren or horn
approaches, passes, and recedes from an observer. The
received frequency is higher (compared to the emitted
frequency) during the approach, it is identical at the
instant of passing by, and it is lower during the recession.

We can applied this to Sirens, astronomy, temperature
measurement, flow measurement, vibration measurement,
radar, velocity measurement, audio, underwater acoustics,
and medical imaging.

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