Ultrasonic system for reliable detection of no, one, or two overlapping sheet materials, preferably papers No teach-in required Function indicators visible from all directions Insensitive to printing, colors, and shining surfaces Material weight from 10 g/m2 up to over 2000 g/m2 Very wide material spectrum, finest papers up to thin sheet metals as well as plastic- and metal foils Perpendicular or inclined sensor mounting relative to the sheet plane possible Programmable