Abstract: A stand-off, non-invasive acoustic detector for monitoring physical activity and/or breathing activity of children and infants. According to a preferred embodiment, the present invention comprises a two-phase output oscillator coupled to an ultrasonic transmitter and microphone. A respective one of the outputs of the oscillator drives the ultrasonic transmitter, through a power amplifier, which sends out an ultrasound wave. The echo return is picked up by the microphone and thereafter band passed to isolate those frequencies falling within 33 to 40 kHz. Such signal is further adjusted for automatic gain control. The resultant echo return signal is compared to the respective other output of the oscillator, the latter also being adjusted for automatic gain control, which are then combined via a summing junction to produce a resultant signal, the latter being utilized to drive a phase comparator and an alarm timer.