The present disclosure relates to an apparatus for determining and/or monitoring a process variable of a medium in a containment, including an oscillatable unit with a membrane, three rods secured to the membrane and extending perpendicularly to a base area of the membrane, a housing, wherein the rods extend into the housing a driving/receiving unit disposed at an end region of the rods and configured to excite the oscillatable unit and transduce mechanical oscillations into a received signal, and an electronics unit configured to produce an exciter signal from the received signal and to ascertain the process variable at least from the received signal. At least one of the rods is secured to the membrane at a site on the base area where the second derivative of the deflection of the membrane from a rest position as a function of the site on the base area is essentially zero.