The invariable wave property in the doppler effect is the:

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. 

10 is the . . .