Nuts & Volts - September 2006

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robot that needs a stiff and strong structure that is lightweight and cheap, then consider making a composite structure from Styrofoam and thin plywood.

SOUNDS OF NEAR SPACE

Now let’s switch gears from ground testing to a near space experiment. In the winter of 1997, I launched what was probably the first camcorder into near space. The scenery was pretty spectacular. But along with the change of scenery, I noticed a hush slowly developing as the near spacecraft climbed higher. The camcorder’s noises were loud at launch but impossible to hear at 85,000 feet. I knew sound could not travel through the vacuum of space, but I didn’t know how sound was affected as the pressure dropped. For example, does sound gradually fade with decreasing pressure or drop off abruptly? How do different frequencies react as the pressure decreases?

As an end-of-semester project, I had some students in my high school electronics class design an audio experiment for near space. Students in my class learn to build circuits and program the BASIC Stamp. I had them use a Parallax application note to build a tone generator. Knowing that at some point it would be impossible to hear the tones, I had my students incorporate LEDs into the circuit to indicate which tone was being generated.

To gather the data, the experiment was attached to the near spacecraft airframe (with a boom) and recorded with a camcorder. The

BASIC Stamp 1 (rev D.) controlling the experiment cycled through the tones in one minute intervals as the balloon rose to an estimated altitude of 85,000 feet. After recovering the near spacecraft — which took five months due to an onboard tracker failure — I played back the video tape. It was obvious the tones were

■ FIGURE 10. The schematic my students used to make the BASIC Stamp tone generator.

not equally affected during the ascent. Higher frequency tones grew quieter sooner than the low frequency tones. (The tape sat untouched until this year when I found a way to analyze its data.)

GREAT RESULTS, NOW LET’S ANALYZE IT

This spring, I visited Dr. Joe Guarino at the College of Engineering at Boise State University. Dr. Guarino’s specialty is acoustical engineering. He analyzes sounds on a PC with a sound card and software called Spectra Pro. To analyze the tone data on my camcorder tape, we connected the audio output of a VCR deck to the inputs of the PC’s sound card. As we played the VCR tape from the mission, Spectra Pro software measured each tone’s frequency and volume.

First, it was necessary to measure the frequencies of the tones generated by the BS-1 since I no longer had a copy of the program my students wrote. Next, we measured the volume of each tone at specific intervals. Since there wasn’t a GPS log, I didn’t know the altitude of the experiment.

The typical ascent rate for my near spacecraft back then was around 1,000 feet per minute. So from the VCR’s displayed time, I estimated the

■ FIGURE 11. The completed experiment. For sitting out in the wild for five months, it’s none worse for wear.

■ FIGURE 12. Spectrum analyzer data. The altitude is estimated based on a 1,000 foot per minute ascent rate. An X indicates no meaningful data could be collected.