download the audio file 6.mp3

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.
• How many main events are happening? Do they seem repetitive?

What is it?

This mystery signal is the sound of 4 whistling "Piccolo Petes" fireworks that were recorded on July 4th.  A Piccolo Pete is an extremely loud noise making pyrotechnic that is legal in many states.  This how it works: Once the powder is ignited, pressurized gas created by the burning fuel exhaust is forced into a whistle like device.  As the fuel burns down, the volume of the tube increases which reduces the pitch of the whistle.  Here is a breakdown of the events in the sound file.

1. (0 seconds) A brief moment of silence before the storm starts.  A lone cricket can be seen chirping away at 7 kHz.
2. (0.8 seconds) The first Piccolo Pete is lit, it starts out with about a 1/3 of a second of wide band hissing.  Also look at the high frequency hole on the far right that is forming because of the MP3 bit allocations.
3. (1.8 seconds) After one second of burn time, the fundamental is finally seen at 2.6 kHz, the 2nd harmonic is the strongest, the 3rd and 4th harmonics are also visible at this moment.
4. (3.2 seconds) The second Piccolo Pete is lit, notice the brief wide band burst of energy between 5 and 6 kHz.  The strong 2nd harmonic at 5.6 kHz lines up almost perfectly with the 3rd harmonic of the first Piccolo Pete.  This is a tricky coincidence (try tracking the traces with the harmonic helper bars).
5. (4.0 seconds) The fundamental of the second Piccolo Pete is visible at 2.3 kHz and an excited crowd starts hootin' and hollerin'.
6. (4.8 seconds) The fundamental for the first Piccolo Pete ends, but it's 2nd harmonic is still strong for the next 4 seconds.  At this point the fundamental and the 2nd to the 8th harmonics of the first Piccolo Pete can be clearly distinguished.
7. (6.2 seconds) Third Piccolo Pete is lit, see the 2nd harmonic at 5.9 kHz.
8. (6.8 seconds) The first Piccolo Pete enters a one second zone of constant burn (steady pitch ~2 kHz) for reasons I can't explain. Possibly the burn rate of the powder changed?  The excited crowd stops screaming.
9. (7.8 seconds) The first Piccolo Pete gets a second wind and it's pitch quickly rises to a constant 2.9 kHz for a duration of one second.
10. (9.0 seconds) The first Piccolo Pete is now spent after about 8.3 seconds of burn time.
11. (9.6 seconds) The fourth Piccolo Pete is lit, see 2nd harmonic at 5.4 kHz.
12. The other Piccolo Petes repeat the same basic cycle.

It is interesting that the 4 Piccolo Petes have burn durations that vary by 2 seconds and minimum constant pitches that vary by 700 Hz.  Not exactly high tolerances, but a Piccolo Pete isn't meant to be a precision piece of equipment.