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Mystery Signal
Mystery Signal of the Month   Jun 5 2006
We've got signal, but what the heck is it?
That's your mission.  Analyze this bébé.

mystery spectrogram                                                                                                                                                          

download the audio file 16.wav.bz2

hints :

  • No need to bunzip2 this file since baudline can automatically uncompress bzip2 files.
  • Setup baudline to be a Web Browser helper application as described in the FAQ.
  • Look and see the interesting things going on in the 250 - 500 Hz region.
  • If you have a subwoofer then slow down the audio to 0.250X with the Play Deck's speed control and listen for sonic details that would otherwise be missed.
  • Use the harmonic bars to determine harmonic relationships.
  • This mystery signal is not palindromic.  What does the shape of the curvature between the frequency transition levels signify?
  • Notice how the strength of the tones at 60 Hz and 120 Hz change when the mystery signal is active.

What is it?

This mystery signal is a common 3-speed table top fan which has 3 plastic fan blades.  The signal begins in silence with the fan turned off.  It is then turned on and cycled up and then down through all three speeds (low -> medium -> high -> medium -> low).  The fan is then turned off, it powers down to the stop state, and the mystery signal ends in silence.

The microphone was positioned behind the fan blades so that the recorded signal was not buffeted by wind noise.  Despite this protected recording position, some low frequency wideband wind noise is still visible.

A strong 120 Hz induction tone is visible while the fan is operating.  Note that this EMF is twice the standard AC line frequency of 60 Hz and it is not an audible signal. 

The start up, speed changes, and shut down transitions all exhibit the usual exponential frequency curves that are common with rotating masses.  This old mystery signal has similar spectral features.

The speed of rotation, in revolutions per minute, can be measured by setting baudline's RPM measurement window to the impeller cycle with 3 blades.  The low, medium, and high speeds are 1048, 1185, and 1346 RPM respectively.  Note that the strongest tone, what looks to be the 2nd harmonic (F2), is in fact the true fundamental.  The phantom fundamental (F0) and the 1st harmonic (F1) are caused by imbalances and imperfections with the construction of the fan's 3 plastic blades.  Both of these tones would be much lower in a higher tolerance device.

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