A device for determining and/or monitoring at least one process variable of a medium, comprising a sensor unit with a unit that can mechanically oscillate, at least one reflection unit (10a, 10b), and a piezoelectric element, which is attached to a membrane, and with electronics, wherein the device is configured to cause the unit that can mechanically oscillate to mechanically oscillate using an excitation signal, receive the mechanical oscillations of the unit that can oscillate and convert them into a first received signal, send a transmission signal, which comprises an ultrasonic signal, and receive this as a second received signal, wherein the electronics are configured to determine the at least one process variable of the medium on the basis of the first and/or second received signal, wherein the unit that can mechanically oscillate has the membrane and two oscillating rods (9a, 9b) molded on to the membrane, wherein the piezoelectric element is attached to the membrane in an area of the membrane facing away from the oscillating rods (9a, 9b); characterized in that at least two reflection units (10a, 10b) are present, each of which is molded on to the oscillating rods (9a, 9b) in an end area facing away from the membrane, wherein at least sections of the two reflection units (10a, 10b) are aligned perpendicular to the oscillating rods (9a, 9b), wherein the reflection units (10a, 10b) are aligned facing each other, starting from the oscillating rods (9a, 9b), in that they run radially inward based on a center point of the surface of the membrane facing the process, starting from the respective oscillating rod, and wherein the reflection units (10a, 10b) are configured to reflect the transmission signal, comprising the ultrasonic signal, back to the piezoelectric element. a sensor unit with a unit that can mechanically oscillate, at least one reflection unit (10a, 10b), and a piezoelectric element, which is attached to a membrane, and with electronics, wherein the device is configured to cause the unit that can mechanically oscillate to mechanically oscillate using an excitation signal, receive the mechanical oscillations of the unit that can oscillate and convert them into a first received signal, send a transmission signal, which comprises an ultrasonic signal, and receive this as a second received signal, wherein the electronics are configured to determine the at least one process variable of the medium on the basis of the first and/or second received signal, wherein the unit that can mechanically oscillate has the membrane and two oscillating rods (9a, 9b) molded on to the membrane, wherein the piezoelectric element is attached to the membrane in an area of the membrane facing away from the oscillating rods (9a, 9b); characterized in that at least two reflection units (10a, 10b) are present, each of which is molded on to the oscillating rods (9a, 9b) in an end area facing away from the membrane, wherein at least sections of the two reflection units (10a, 10b) are aligned perpendicular to the oscillating rods (9a, 9b), wherein the reflection units (10a, 10b) are aligned facing each other, starting from the oscillating rods (9a, 9b), in that they run radially inward based on a center point of the surface of the membrane facing the process, starting from the respective oscillating rod, and wherein the reflection units (10a, 10b) are configured to reflect the transmission signal, comprising the ultrasonic signal, back to the piezoelectric element.