Ultrasonic detectors , as the word implies, use sound to detect movement. The detectors are active in that they are sending out and receiving ultrasonic signals. This type of detector is often used in places where conditions do not allow a PIR to be used. They cannot be used in a shop that has posters that swing in the ventilation system or doors that may move in a draught. As with the PIR, these detectors have their uses and their positioning is critical for reliable use.
Ultrasonic detectors are most effective when an intruder walks toward or away from the detector.
The MX detector is something different in that it uses microwaves as does a radar. They are rather more expensive than the other two types of movement detectors, and use more power. They are used in circumstances of special demand, and a great deal of consideration must be used before they are permanently placed. The MX detector, because it is based on the same principles as radar, has the ability to see through walls and furniture etc. Therefore if it is placed, for example, in a room at the side of a building, then there is an even chance that it will pick up everyone who walks past the outside of that building.