The ultrasonic waves (40 kHz) are actively emitted from the sensor and spread throughout the space it covers. They permeate into every last corner, enveloping objects that are present in the room. The sensor identifies movement in the room even if there is no visual contact between sensor and person. The signal is evaluated on the basis of the Doppler principle. It is this that makes ultrasonic sensors ideal for open-plan offices and rooms with stud walls. Ultrasonic waves do not penetrate thin walls and are highly sensitive. And therefore particularly recommended for rooms in which sedentary activities are performed.