Respuesta :

The Doppler effect describes the shift in the frequency of a wave sound when the wave source and/or the receiver is moving. If the wave source and/or the receiver is moving than there is an increasing or decreasing in the distance between the source and the receiver and that causes the received frequency of a source  to differ from the sent frequency
The invariable wave property in the Doppler effect is the velocity. 

The answer to you question is velocity

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