Sounds in Near Space
■ FIGURE 13
Altitude (feet estimated)
5. 6 kHz
3. 9 kHz
played. Several times we backed up the tape and
try clicking again. Even when we clicked the
mouse during a tone, we often did not measure
the tone at its loudest volume.
THE DATA COLLECTED
From measurements taken at ground level,
we determined the seven tones had these
10. 5 kHz
5. 6 kHz
3. 9 kHz
altitude of the balloon each time we measured the tone
volumes. I decided to measure each tone every 10,000 feet
or at 10 minute intervals. This limited the amount of data
collected, but also limited the burden I was placing on Dr.
Guarino. One of the problems we experienced was that
Spectra Pro only measures sound volume at the instant the
mouse is clicked. So if our timing was off — and often it was
— we didn’t get the volume of the tone while it was being
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The noise floor for the video tape was -125 dB. This
means that when there were no tones on the tape, the
background noise had a volume of -125 dB. This also means
that tone volumes couldn’t be measured once they fell
below -125 dB.
From the information above, I generated the a chart
with Excel (shown in Figure 13).
First, notice that the volume of each frequency
decreases with increasing altitude. Also notice that the
tones are not equally affected. You can see that the lower
the frequency of a tone, the higher the altitude it could be
recorded by the camcorder. I was expecting smooth
changes in volume and suspect my data would be
smoother if we timed our sampling better.
What does this data mean and what influences the
maximum altitude that a tone can be heard at? I don’t
know. I’ve charted the speed of sound and the mean free
path of molecules as a function of altitude, but I can’t find
a correlation. Perhaps if I sampled the data better, a relationship would become apparent. Like many good science
experiments, I’m left with more questions than answers. So,
perhaps it’s time to redesign this experiment and fly it again.
IMPROVEMENTS FOR NEXT TIME
Here’s how I plan to improve the design of this
experiment before its next flight. First, I’ll create a better
tone generator. To do this experiment right, I need a wider
range of frequencies. The one inch diameter speaker used
in the original experiment was ineffective at generating low
frequency sounds. So I’ll use larger speakers for lower
frequency tones and smaller speakers for higher frequency
Second, I need to lower the noise level. I plan to use
Styrofoam to build an enclosure that blocks background
noise and shields the tone generator and sound recorder