This skin is about the Doppler Effect.
The Doppler Effect refers to the change in wave frequency during the relative motion between a wave source and its observer.
It is named after the Austrian physicist Christian Doppler, who described the phenomenon in 1842.

The Doppler Effect will cause the source to sound higher in pitch when it is approaching than when it is receding.
A common example of Doppler shift is the change of pitch heard when a vehicle sounding a horn approaches and recedes from an observer. Compared to the emitted frequency, the received frequency is higher during the approach, identical at the instant of passing by, and lower during the recession.
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