Ultrasonic Motion Sensors
Ultrasonic Motion Sensors

Some homeowners choose to use motion detectors as a security measure. Motion detectors survey a certain area and activate other measures, such as alarms, if they detect motion. One such device, ultrasonic motion sensor, detects motion using sound waves. As movement disrupts air and sound waves, these sensors are able to sound alerts of intrusions. Two types of sensors fit this category, active and passive. Active sensors emit sounds at pitches beyond the capacity of human ears to hear. Passive sensors consist only of receivers that listen for sounds. They are among the most sensitive of detection systems. Thus, they are also among the most expensive.

Active Sensors

Active ultrasound sensors emit sound waves from quartz-crystal transducers. The waves strike objects within the field of detection and as long as there is no movement the waves are not disrupted. However, when movement occurs the sound wave is disrupted and is reflected back to the system's receiver. The receiver then relays the signal to the electronic control unit, and depending on its sensitivity and calibration, the electronic control unit determines if the signal disruption warrants inordinate movement. If so, it then relays the signal to the alarm component which triggers the alarm. The sound waves create a complete field of detection, so that any point within the field falls within its range. The field of detection weakens with distance. However, motion is generally detectable up to a distance of about forty feet.

Passive Sensors

Passive sensors operate on the principle of sounds such as breaking glass or metal striking metal to trigger alarms. These sounds produce waves detected by the sensors which, like the active sensors, relay them to electronic control units to determine if the sound wave pattern falls within established normal parameters. Since skilled burglars know how to pass through space without making highly audible sounds, many people use these sensors in conjunction with passive infrared sensors to increase sensitivity. Newer models combine both technologies within one system.


Most systems have a life span of approximately twelve to fifteen years; however, control units may have life spans as short as six to as much as ten years. High temperatures and humidity may cause premature system deterioration.


The sensitive detection abilities of these sensors offer obvious advantages to consumers who wish to monitor property. Nevertheless, this sensitivity can also cause many false alarms. Highly sensitive active detectors may trigger false alarms when detecting innocuous objects such as passing insects or birds. Gusts of wind or vibrations caused by airplanes passing overhead may also trigger false alarms. Also, ultrasound cannot penetrate solid objects such as furniture or boxes and these areas cause gaps in the sensor's detection field. In addition, precise, slow movements by a burglar may not trigger an alarm. Passive detection units may falsely detect pipe discharges and ringing telephones.

On the other hand, some systems are calibrated to not detect such noises but they also do not detect drilling through door knobs. Objects such as furniture may also obfuscate sounds which the sensor may not detect. These systems utilize advanced technology and are among the most thorough of detection systems. An active system detects everything within its range. Nevertheless, they are expensive, and under some conditions prone to false alarms. If you live in an area with an unusual amount of noise such as an airport, industrial area, or near a construction zone then these systems may not suitable for your needs. Also, passive systems do not provide complete detection in areas with large objects and in those cases another type of detection system may be required to compliment the ultrasound system.