The ultrasonic waves are actively emitted from the sensors and completely fill the space they cover. Highly sensitive and able to detect minor movements, the ultrasonic sensors are effective for low motion activities, which is why they are particularly recommended for rooms in which sedentary activities are performed.
Ultrasonic sensors also detect a larger area than a PIR sensor, meaning fewer sensors would be required.
However, they cannot be mounted higher than 4.2 metres with the sensors effective range declining proportionally to partition height. They are very susceptible to high levels of vibration of airflow and not adjustable so not ideal for areas that require selective coverage.