<-----Original Message-----
<From: phys-l@lists.nau.edu: Forum for Physics Educators
<[mailto:PHYS-L@lists.nau.edu]On Behalf Of paul o johnson
<
<You are, indeed, right, Ken. The max. sampling rate for the Pasco device is
<120 Hz. That leads to a max. distance of 8 meters, I believe.
<
I don't understand that. The sound must make a round trip between sensor and
object, and this must be completed before the next pulse is emitted.
At 1/120 second, and a speed of sound of about 330 m/s, we've a distance
traveled of 330/120 m, or about 2.7 m. Cut that distance in half for the round
trip, and we've a little over a meter for the maximum distance of an object
away from the detector if it is running at 120 Hz.
That's assuming no dead time before a pulse is emitted, so the real situation
is worse.
This is one reason one rarely operates motion detectors at such a high
rate--even 100 or 80 Hz is pretty limiting.
The minimum distance detectable by motion detectors is determined by the time
necessary for the diaphragm to stop ringing. By design, the dead time is made
over-long so as to never have interference from the ringing. Most units can be
adjusted to reduce that dead time, and thus reduce the minimum distance.
JEG
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