The invention relates to a device for determining and/or monitoring at least one process variable of a medium (20), having at least one unit (1) that can oscillate mechanically and that comes into contact with the medium (20) at least partially, having at least one transducer unit (2), which induces the unit (1) that can oscillate mechanically to oscillate mechanically and which receives mechanical oscillations from the unit (1) that can oscillate mechanically, and having at least one housing (3). According to the invention, at least one bounding element (10) is provided, and the bounding element (10) is designed and arranged in such a way that the bounding element (10) at least partially surrounds the unit (1) that can oscillate mechanically and the resulting reduction of the volume surrounding the unit (1) that can oscillate mechanically increases the measurement sensitivity of the device. The invention further relates to a bounding element (10).