Device for determining and/or monitoring at least one process variable of a medium, comprising a mechanically vibrating unit, a first piezoelectric element serving as a drive/receiver unit and an electronics unit, wherein the drive/receiver unit is designed to excite the mechanically vibrating unit to mechanical vibrations by means of an electrical excitation signal, to receive the mechanical vibrations of the mechanically vibrating unit and to convert them into a first electrical reception signal, wherein the electronic unit is designed to generate the excitation signal starting from the first received signal and to adjust a frequency of the excitation signal in such a way that a predeterminable phase shift is present between the excitation signal and the first received signal, wherein the mechanically vibrating unit comprises a diaphragm, and wherein a first oscillating element and a second oscillating element are formed on the diaphragm, characterized in that that a surface of the membrane facing the process is curved, that the surface of the membrane is convex at least in sections, that the device further comprises at least a second piezoelectric element, in that the first piezoelectric element is arranged at least partially in the first oscillating element and the second piezoelectric element is arranged at least partially in the second oscillating element, in that the first piezoelectric element and the second piezoelectric element are designed to excite the mechanically vibrating unit to mechanical vibrations by means of the excitation signal, to receive the mechanical vibrations of the vibrating unit and to convert them into the first reception signal, that the first piezoelectric element is designed to emit a transmission signal, which is an ultrasonic signal, in that the second piezoelectric element is designed to receive the transmit signal in the form of a second receive signal, and in that the electronics unit is designed to determine the at least one process variable of the medium on the basis of the first received signal and/or on the basis of the second received signal. wherein the drive/receiver unit is designed to excite the mechanically vibrating unit to mechanical vibrations by means of an electrical excitation signal, to receive the mechanical vibrations of the mechanically vibrating unit and to convert them into a first electrical reception signal, wherein the electronic unit is designed to generate the excitation signal starting from the first received signal and to adjust a frequency of the excitation signal in such a way that a predeterminable phase shift is present between the excitation signal and the first received signal, wherein the mechanically vibrating unit comprises a diaphragm, and wherein a first oscillating element and a second oscillating element are formed on the diaphragm, characterized in that that a surface of the membrane facing the process is curved, that the surface of the membrane is convex at least in sections, that the device further comprises at least a second piezoelectric element, in that the first piezoelectric element is arranged at least partially in the first oscillating element and the second piezoelectric element is arranged at least partially in the second oscillating element, in that the first piezoelectric element and the second piezoelectric element are designed to excite the mechanically vibrating unit to mechanical vibrations by means of the excitation signal, to receive the mechanical vibrations of the vibrating unit and to convert them into the first reception signal, that the first piezoelectric element is designed to emit a transmission signal, which is an ultrasonic signal, in that the second piezoelectric element is designed to receive the transmit signal in the form of a second receive signal, and in that the electronics unit is designed to determine the at least one process variable of the medium on the basis of the first received signal and/or on the basis of the second received signal.