Turbulent flows of gases and liquids, e.g. in pipes or valves, can be monitored with permanently installed structure-borne sound probes (contact probes) as well as air-borne sound probes. In addition, structure-borne sound probes are also suitable for processes in which friction is responsible for the generation of ultrasound (e.g. bearing testing). With the T20 ultrasonic transducer, the ultrasonic experts are launching a structure-borne (contact) sound probe for stationary condition monitoring of machines, systems, and processes. In combination with the stationary sensor box S-SB10 and a fixed installation of ultrasonic transducers at an inspection point, short-term level changes as well as long-term trend developments can be observed. Based on changes in ultrasonic levels, it is possible to make statements about the condition of the plant or process. For recurring measurements with recording of measurement data, the ultrasonic probe can be fixed with a magnetic base or universal magnet. For continuous monitoring or fixed mounting within a safety cover/zone, the probe is screwed on with a grub screw.