A motion detector sends out ultrasonic pulses and listens for beat frequencies in the reflections to identify when objects are moving. If the motion sensor produces an ultrasound pulse with a frequency of 120 kHz and should be able to observe motion down to 0.36 m/s, what is the lowest beat frequency that it should be able to detect? The speed of sound is 330 m/s in this situation. Hint: Remember that in reflections we have to apply Doppler twice -- once for what the object "hears" and once for the sound coming back to the object to the receiver. 119,738.2