Ultrasonic sensors are small devices used to detect the presence of intruders and other foreign objects. These can discreetly be placed in courtyards and inside homes to detect the presence of moving objects alien to the surroundings. They operate at frequencies in the range of twenty-five to forty kilohertz, which is way above the human hearing range.

The basic mechanism used in ultrasonic sensors activates a quartz crystal that emits ultrasonic sound waves in the surrounding areas. The detector system senses the frequency of reflected waves to detect any slight changes that might occur due to a moving object technically known as the Doppler effect.

These devices are designed to produce ultrasonic sound waves in a continuous fashion without any blind spots or gaps in the coverage pattern. This makes it possible to detect small gestures such as hand motion, which can be detected at a distance of twenty-five feet, arm and body movement at thirty feet, and full body motion at forty feet depending on the type of sensor used. Advanced ultrasonic proximity sensors used for spy surveillance generate a highly focused beam of ultrasonic sound that can easily access tight spaces and openings ranging from .5 inch to four inches long in diameter.

Some of these devices are equipped with external touch function that allows the user to remotely program the best scanning range for the application. They may also have device-locking features that prevent unwanted output programming changes. Apart from detecting human movement, they also have the capability to detect any material that can bounce back sound such as liquids and clear substrates like plastic and glass.

Ultrasonic sensors are also used in different industrial sectors such as pharmaceutical, medical, food, cosmetic, and household & personal care manufacturing. They have enabled the detection of minor flaws in products during the production process, which if left unchecked can damage the brand value of a product or company.