18 kHz since they are generally inaudible [1]. Additionally,
the higher the frequency, the greater the shift for a given
velocity, which makes it computationally easier to estimate
motion at a given resolution. The upper bound is largely a
function of most laptop and phone speaker systems only
being capable of producing audio at up to 22 kHz. Fortunately,
we do not need much higher frequencies to sense the
relatively coarse gestures we are targeting.