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• Uses either ultrasonic waves or microwaves to detect motion. When it detects motion, it triggers an alarm.
• Generally plugs into a standard electrical outlet, so they’re easy to install.
• Ultrasonic detector waves only go as far as the wall of the room, while microwaves penetrate walls and windows.
• Some ultrasonic alarms use a narrow sonic beam that must be bounced off a hard surface. Here, the intruder must break the beam to be detected.
• Other ultrasonic alarms use a wide beam that fills the room and detects motion anywhere in the trap zone.
• The most important consideration with this type of alarm is avoiding false alarms. These can be caused by air conditioners or drapery moving above a hot air register. Better systems have built in circuitry that does not respond to these types of signals.
• Some systems have extra repeater alarms or satellites. These can be plugged in anywhere in the home to add extra noise and relay the warning to remote rooms.