An apparatus for establishing and/or monitoring a predetermined filling level in a container is described, which apparatus can be used in a wide range of applications, that is to say in many different media, which apparatus comprises: a housing (1), an oscillating element for projecting into the container, a diaphragm (5), which is fixedly clamped into the housing (1) at its edge and to which the oscillating element is fastened, and an electromechanical transducer for exciting the diaphragm (5) to produce oscillations in dependence on a transmission signal (S) present at the transducer and for receiving and converting oscillations thereof into an electrical reception signal (E), in which a percentage interval between a resonant frequency fr and an antiresonant frequency far divided by the resonant frequency fr exceeds 20% when the oscillating element oscillates in air.