Which occupancy sensor uses reflected sound waves and listens for a phase shift in the reflected sound returning to sense the presence of people?

Which occupancy sensor uses reflected sound waves and listens for a phase shift in the reflected sound returning to sense the presence of people?

• A. Infrared sensor
• B. Ultrasonic
• C. Microwave sensor
• D. Passive infrared

Answer: (B)

Ultrasonic sensors emit high‑frequency sound waves and listen for their echoes. They detect presence by analyzing how the reflected sound changes, particularly a phase shift in the returning wave caused by movement. The controller interprets this phase shift to determine occupancy. This works with sound, not light or heat, which is why infrared-based sensors (which detect heat) and passive infrared sensors (which don’t emit anything) don’t fit. Microwave sensors use radio waves, not sound, so they rely on different detection principles. So the method described—reflected sound waves and a phase shift in the return—matches ultrasonic occupancy sensing.

Ultrasonic sensors emit high‑frequency sound waves and listen for their echoes. They detect presence by analyzing how the reflected sound changes, particularly a phase shift in the returning wave caused by movement. The controller interprets this phase shift to determine occupancy. This works with sound, not light or heat, which is why infrared-based sensors (which detect heat) and passive infrared sensors (which don’t emit anything) don’t fit. Microwave sensors use radio waves, not sound, so they rely on different detection principles. So the method described—reflected sound waves and a phase shift in the return—matches ultrasonic occupancy sensing.