Motion Detection and Feature Extraction: The frequency
vectors have a per-bin resolution of 21.5 Hz. With a pilot
tone of 20 kHz this translates to detecting movements as
slow as 18.5 cm/sec. In practice, we have found that the
bandwidth of the pilot tone itself with no motion is ~80 Hz,
which can vary from 60-120 Hz (1-3 bins on either side of
the tone) depending on the quality of the sound system.
Thus, we consider a “motion event” to occur when there is
a frequency shift bandwidth of 4 or more bins. We have
found that this threshold allows sufficiently slow movements
of the hand to be detected while ignoring false positives
due to variations in the bandwidth.