Helicopter approach and departure - Doppler Effect on RS1D

Here’s a great demonstration of Doppler Effect showing the approach and departure of a helicopter. The view is spectogram and shows a clear frequency decrease at flyover. This was recorded using a RS1D 4.5 Hz vertical geophone. The main train goes from about 28 Hz to 12 Hz. You can see a higher order train coming down from 50 HZ right at flyover.


I see Military choppers fly by my Shake and Boom… Here’s a sample. I wonder if there is a difference in the blade configuration that shows fewer lines? Perhaps there is some difference with the speed of the craft and the doppler difference between the blade angle to/from its velocity?