download the audio file 1.wav.gz

hints :

• Setup baudline to be a Netscape helper application as described in the FAQ.
• Ignore the 60 Hz AC power leakage and its harmonics (120 Hz, 180 Hz).
• Turn on the 60 Hz notch filter in the play deck.
• Try playing the mystery signal at different speeds, slow it down or drop it an octave.
• Measure the harmonic relationships, what are the fundamentals?

What is it?

No, it's not a boomer out of Murmansk.  It is a very big airplane with twin screws.

This plane was on its landing approach flying very low and slow as it passed overhead.  The spinning propellers cutting through the air are the cause of the rich harmonics, and the decreasing frequency is explained by Doppler shift.  The fundamental drops in frequency from about 19 Hz to 11 Hz with the true fundamental being somewhere in the middle.  Having only the 4th and higher harmonics being audible is very characteristic of large propeller driven planes.  The fundamental frequency is strangely low for an airplane but it could be due to the throttling down and scrubbing off of excess speed. 

The two pairs of close tone lines are caused the two propellers (twin screws) spinning at slightly different speeds.  From the 80 to the 60 second mark the pilot slowly lowers the throttle which explains the brief decreasing exponential curve shape to the harmonics.  At this point something really bizarre happens; the twin screws "criss and cross" several times which  means the relationship between their rotational speeds is changing from positive to negative back to positive several times.  The twisting and intertwining of the twin screws could of been due to the changing air/fuel mixture, a drastically changing engine load, air turbulence, or a number of other things. Typically dual turbo-prop planes don't have the twin screw harmonics signature seen here.  This is because the pilots sync the speeds of both engines in order to prevent annoying frequency beating.