Acoustic Position Measuring Device

Charles Pax charles.pax at
Tue Aug 19 16:49:01 CEST 2008

Hey, gang, here's an idea: Use the Freerunner as an acoustic measuring
device. (This is just a brain dump and not super detailed.)

I got the idea while driving from Maine to New Jersey on Saturday during a
discussion with my brother about an article I had read [1]. We were trying
to think of potential applications that can replace physical objects. He
jokingly said, "a ruler." We first laughed at the image of someone using the
screen to measure some tiny object, but then about an hour later I began to
puzzle over the problem. I teach high school physics, so I naturally thought
of all the different Pasco and LabPro sensors that can be hooked to the
Freerunner via USB. When I came to the motion sensor [2] I remembered our
ruler joke and figured the Freerunner can be a ruler or measuring tape if
the motion sensor was connected to it. But then I remembered how the motion
sensor works; the sensor emits a sound pressure wave (a chirp), the chirp
bounces off an object, and the sensor senses the returning chirp. We don't
need a sensor, the Freerunner already has the necessary hardware: a speaker
and a microphone.

1) Through the speaker emit a chirp at an initial clock reading (t_i) and
lasting a known time interval (T_i).
2) The chirp is reflected off an object.
3) Through the microphone sense the returning chirp and measure the clock
reading (t_f) and the chirp time interval (T_f).
4) Find the distance (d) to an object using the relationship
        d = (t_f - t_i)(v)
   where v is the speed of sound.
5) Determine the object's speed (s) relative to Freerunner (s = 0 for a good
distance measurement) using the relationship
       s = (v * Ti_i)/(T_f) - v

The microphone would sense the chirp as it travels directly from the speaker
to microphone, so some small time interval should pass before the
application starts using microphone data as being bounced off an object.
However, given that the speaker-to-microphone distance is known, we may be
able to calculate the speed of sound using time interval during the chirp's
travel directly from speaker to microphone.

Since we can measure the distance of an object from the Freerunner, we can
also get velocity and acceleration just like the motion sensor [2].

I won't have a Freerunner until they are back in stock and the NYC sales
group picks up another ten pack, so I can't do any real work yet. However,
I'll draw up some pseudo code after I get some comments on this.

I have a few questions
1) Are the any technical obstacles to this idea?
2) What is the frequency range of the speaker?
3) What is the frequency range of the microphone?
4) Any other comments of ideas?

-Charles Edward Pax

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