I recently flew as a passenger on a Boeing 717-200 aircraft similar to the one shown in the image. This aircraft has two Rolls-Royce BR700 engines, with the following specifications:
Maximum Engine Rotational Speeds (Both Engines)
N1 Low Pressure Turbine = 6,195 RPM (103 Hz)
N2 High Pressure Turbine = 15,898 RPM (265 Hz)
I made an audio recording from inside the cabin during take-off and climb-out.
The sound file Fourier transform for a 10-second segment is shown in the image.
The first peak is at 88 Hz, which is 85% of the maximum N1 speed.
The second peak is at 129 Hz and is unidentified.
Most of the higher frequency peaks are integer multiples of 88 Hz.
Complete audio file: Boeing_717_200.mp3
The Fourier transform was taken from 40 to 50 seconds into the recording.
A “buzz saw” sound occurs due to shock waves at the turbofan blade tips which have a supersonic tangential velocity.
– Tom Irvine