I recently flew as a passenger in a CRJ-200 (aka CL-65) jet similar to the one shown in Figure 1.
This aircraft has two General Electric CF34-3B1 turbofan engines, as shown in Figure 2.
Here are the rotational speeds for each rotor.
N1 Fan Speed 7300 RPM (122 Hz)
N2 Core Speed 17710 RPM (295 Hz)
I made an audio recording from within the aircraft cabin nearing the end of climb-out, after hearing some distinct sine tones against the background random noise. The audio file is: CRJ200.wav
A Fourier transform of the sound file is shown in Figure 3.
A spectral peak occurs at 113 Hz, which is 93% of the N1 Fan speed.
A pair of spectral peaks occur at 277 and 278 Hz. These are about 93% of the N2 Core speed. The 1 Hz difference could be due to the N2 rotors of each engine being slightly out-of-sync with one another.
The other peaks remain unidentified.
Again, this recording was made from inside the cabin. So the fuselage walls would have attenuated some of the engine-generated acoustic energy, particularly at higher frequencies.
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See also: Bombardier
– Tom Irvine