download the audio file 5.wav.gz

hints :

• Setup baudline to be a Netscape helper application as described in the FAQ.
• 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.
• Use baudline's harmonic helper bars and delta measurement bars to determine the harmonic relationships.

What is it?

This mystery signal may sound like a very large and very angry bumble bee, but it's not.  Slow it down to a quarter speed and  you will hear the true infernal racket of dual pneumatic jack hammers.  I had the pleasure of having the driveway just outside my window smashed into little pieces by these incredibly loud tools of destruction.  So in standard SigBlips fashion let's analyze the components and break this signal down (heh heh pun intended).

The steady state 50 Hz fundamental with an even stronger 2nd harmonic at 100 Hz is the large gas powered air compressor that powers the jack hammers.  Notice how the tone wavers several Hz up and down as the jack hammer load changes.

The pulsed jack hammer bursts have a fundamental of around 20 Hz with strong harmonics all the way up to and past the Nyquist rate of 250 Hz. Considering that concrete is being violently crushed, one would expect loud broad band noise, but instead the sound is a rather  pure tone with strong harmonic content, very much like a human voice making the "mmmmm" sound.

Finding evidence for the existence of two jack hammers is not obvious.  One jack hammer was much closer to the microphone making it about 16 dB louder thus masking the quieter one.  The short in duration and lower amplitude blips at the 43.5 and 48 second marks have a very different harmonic signature (see average spectrum display below).  The burst from 20 to 25 seconds is interesting in that you can see both jack hammers at work at the same time.