Actually, the expression itself is inaccurate, as a rule, an ultrasonic sensor is made up of a transmitter and a receiver, both, by the way, are quite similarly constructed. The frequencies lie in the inaudible and of course also invisible range of 20kHz to 10MHz. In the older generation of ultrasonic sensors a magnetic moving coil, together with a membrane was used, the newer ones use multiple layers of Piezo crystals.
At a velocity of 350 m/sec. the emitted waves are reflected off objects, at best with one level surface, they are then picked up and highly sensitvely evaluated by the two-stage transistor amplifier. In the automobile field, the time, e.g., in the case of distance warning systems, must be precisely evaluated, as far as anti-theft alarms are concerned, precision is not that important because only a certain area is monitored.
Indeed, very important in the anti-theft monitoring, is the avoidance of pseudo echoes, because two transmitters are often placed opposite and facing one another, the one could erroneously assume that the waves from the other are a reflection. It is important e.g., for the interior monitoring, that the sensitivity can be adjusted by varying the resistance enlargement of the transistor amplification. 01/12