I recently flew as a passenger in a CRJ900 jet similar to the one shown in Figure 1.
This aircraft has two GE CF34-8C5 engines, as shown in Figure 2.
This engine has two high pressure axial flow stages that drive the N2 compressor, and four low pressure stages that drive the N1 fan.
Here are the rotational speeds for each rotor.
Low Pressure Rotor (N1):
Maximum Takeoff: 7,360 RPM
Normal Takeoff: 7,360 RPM
Maximum Continuous: 7,360 RPM
High Pressure Rotor (N2):
Maximum Takeoff: 17,710 RPM
Normal Takeoff: 17,710 RPM
Maximum Continuous: 17,470 RPM
I made an audio recording from within the aircraft cabin during cruise, after hearing some distinct sine tones against the background random noise. The audio file is: crj900.wav
A Fourier transform of the sound file is shown in Figure 3.
The highest peak occurred at 104.3 Hz with a 2x harmonic at 208.6 Hz. The peak at 104.3 Hz is equivalent to 6258 rpm, which is 85% of the N1 maximum continuous speed.
Note that turbofan engines are typically operated somewhat below the maximum speed in order to achieve optimal fuel efficiency, which is referred to as best Thrust Specific Fuel Consumption (TSFC) . Preserving engine life is another reason for running below maximum speed.
The spectral peak at 104.3 Hz is thus plausibly due to the N1 fan rotor speed.
The second highest peak occurs at 252 Hz (15,120 rpm). This is 85% of the N2 core rotor speed.
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