Acoustic distance measurement test results

[Benjamin M. Schwartz]

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Joyride builds currently include my Acoustic Tape Measure activity, designed to
turn any pair of laptops into a tool for measuring distance.  Today, I decided
to test the maximum range.  I used two B4's running clean installs of joyride 289.

I went to the MIT football field, because it is a large, flat, open space with
clearly labeled distance markings.  Unfortunately, there were no markings
visible on the field, so I made measurements on the adjacent long-jump track,
which is labeled in feet.

Measurement worked perfectly up to 30 m (100 ft), with at most 1 cm of
variability, and usually none. Measurement worked unreliably up to 40 m, giving
spurious answers except during lulls in the wind. (Almost all microphones record
high-amplitude noise in the presence of wind.)

I have several ideas for increasing range (or equivalently, improving noise
tolerance).  However, I did not make any attempt to implement them, because it
was too cold out for programming.

I also encountered some difficulty when sharing the activity over the mesh at
distances greater than 25 meters.  This might be because the default mesh
frequency (Channel 1) is the same as MIT's pervasive wireless network.  After
switching to mesh channel 11, I had no difficulty sharing over distances up to 40 m.

- --Ben Schwartz
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