Sound waves were emitted from a sensor and the resulting reflected sounds were analyzed to detect changes. Ultrasonic sensors engage when they detect a change in ultrasonic frequencies caused by motion in a space. It is only through movement that an ultrasonic sensor is triggered.

There are limitations that affect their use in home security settings. Jingling keys and ringing telephones may set off the alarms. Since ultrasonic sensors don’t detect very fast or slow moving objects, an intruder could hide behind an object and move across the sensor’s viewable area and not set off. The disadvantage of ultrasonic sensors is that they don’t work well in rooms with wall to wall carpeting and drapery. The sound waves from the transmitter get absorbed by these materials and are too weak to return to the sensor. Ultrasonic sensors were used in alarms system back in the 70′s and because of the constant false alarms caused by this sensor; it has been rendered obsolete and replaced by a more reliable sensor such as the infrared sensor.

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