Finding rotation rate from sound files

So how fast does the copter rotate? One can hear the characteristic volume variations in the videos, so:

• extract sound from video
• extract volume/envelope
• autocorrelate that
• plot

A bit of fiddling with Octave, and here it is:

It turns out the fastest rotation has a period of about 0.24s, but there the AoI is too low so it doesn't lift.

When it does lift, it rotates with a period of about 0.29s, that is 3.4Hz or 207 RPM.

The code is here.