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.