Neither of these apply to passing helicopters. A hovering 'copter has no Doppler shift, nor can a helicopter fly at the speed of sound (the forward moving blade tip would be faster than the speed of sound) and the other side would have very little lift – and zero lift when the first side was at Mach 2!
However, these extreme cases help show things like the greater increase of the arrival frequency compared to the decline of the departure frequency.