An improved alarm system for monitoring movement through a passageway defined by opposing sidewalls comprises a lower passage indicator disposed toward the lower ends of the sidewalls, an upper passage indicator disposed toward the upper ends of the sidewalls and a control mechanism configured to sound an alarm if one of the passage indicators indicates passage through the passageway and the other passage indicator does not. In a preferred embodiment, the alarm system is configured as a pool alarm and each passage indicator comprises an emitter/receiver that emits an infrared beam toward an opposing reflector to provide a lower beam and an upper beam. If only the lower beam is interrupted, indicating a child passing through the passageway, the control mechanism will generate an alarm signal. If both beams are interrupted, indicating an adult passing through the passageway, the control mechanism will not generate the alarm signal.