Ultrasonic sensors emit high-frequency waves in the range of 25 to 40 kilohertz, above the range of normal human hearing. These waves bounce off objects in the room and return to the sensor. Ultrasonic sensors usually can detect small movements and do not require a direct line of sight to occupants, but wind-blown curtains or papers can trigger the sensor. Manufacturers of sensors offer an adjustable time-delay feature that allows users to specify how long the lights stay on after motion is detected. A long time delay reduces the chance of the sensor turning off the lights while someone is still in the room, but wastes energy if the room is infrequently occupied.
Occupancy sensors are usually either wall or ceiling mounted.
Most wall-mounted sensors are designed to replace standard wall switches and are as simple to install as a dimming switch. Wall-mounted units suit smaller rooms such as private offices, copy rooms, and bathrooms. A ceiling-mounted sensor may be wired to a remotely located control module that, in turn, can be wired to several lighting circuits. Units that are designed for mounting in the corner of a room or on the wall near the ceiling also are available. Although manufacturers report a specific coverage area for an occupancy sensor, the actual area within which a sensor can detect motion depends on several factors including the mounting location, room geometry, mounting height, furniture layout, the sensor's sensitivity setting, and the type of motion.