Help Needed… Aircraft Spectrum Analysis

Unknown Spectral Feature - Air Tractor Fixed Wing Aircraft

My search for Doppler shifted sounds has resulted in a number of recordings of aircraft. As I studied those recordings for the Doppler shift, I noticed that there was also another feature in the spectrograms that I didn’t understand. There is a shallow bowl shaped feature centered on the time of closest approach, ie when the aircraft was roughly overhead. The “bowl” seems to be formed by harmonic content in the signal. The frequency increases not only as the aircraft is approaching, as one would expect for a Doppler shift, but also as it moves away. The two spectrograms accompanying this post, for the Air Tractor fixed wing and the Chinook helicopter, show the feature pretty clearly.

I have thought about refraction due to the temperature lapse rate as a function of altitude but the equations don’t show any frequency dependence. I considered source directivity but I see it in two very different aircraft.  If you listen carefully to an airplane or helicopter flying over you can hear a frequency component that is increasing as the craft flies away from you…it seems to be a real phenomenon.  I am stumped!

Please comment if you know what causes this feature or if you have a suggestion as to what it might be.


Unknown Spectral Feature - Chinook Helicopter

2 thoughts on “Help Needed… Aircraft Spectrum Analysis

  1. hello
    about aircraft spectrum analysis, the reason of the modulation show on the spectrogams is due to the interference between the different path: direct on microphone position and the same reflected from the ground. According to the relative position of the airplane fly over and the fix position of the microphone at some meters from the ground, the noise spectrogram assume this paricular shape. To avoid this problem try to position the microphone directly on the ground.

  2. Alberto,

    Thank you for your comment. I never considered multi-path interference but my son reminded me that he suggested that as a possibility. I’ll give that a try.


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