|We've got signal, but what the heck is it?
That's your mission. Analyze this bébé.
download the audio file 1.wav.gz
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,
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.