An improved alarm system 10 for monitoring movement through a passageway 12 defined by opposing sidewalls 20/22. The alarm system 10 has a 5 lower passage indicator 34 disposed toward the lower ends 52/54 of the sidewalls 20/22, an upper passage indicator 36 disposed toward the upper ends 56/58 of the sidewalls 20/22 and a control mechanism 38 configured to sound an alarm 72 if the lower passage indicator 34 indicates passage through the passageway 12 and the upper passage indicator 36 does not. In one embodiment, each of the passage 10 indicators 34/36 comprises transducers 80/82/86/88 that are configured to direct and receive sound waves, such as ultrasound waves 84/90, and provide lower 84 and upper 90 ultrasound waves that act as lower and upper barriers.