Disclosed is a dentists' preparation instrument (1) comprising a turbine (4) for driving a tool (3) using compressed air, and a rotor (6) that is arranged in a turbine compartment (5), rotates about a longitudinal axis (7), and has blades (11) extending to a face (13) of the rotor (6). The turbine compartment (5) has a wall (10) opposite the face (13), and a discharge port (12) for the compressed air in the direction of a returning air duct (8). The discharge port (12) is disposed in such a way that at leasts parts of the blades (11) swipe past the discharge port (12) as they rotate about the longitudinal axis (7). The turbine compartment (5) also has a disruptive contour chamber (15) that adjoins the face (13). The face (13) of the blades (11) of the rotor (6) that faces the disruptive contour chamber is open towards the disruptive contour chamber (15). Inside the disruptive contour chamber (15), a disruptive contour (9) is formed on the wall (10) opposite the face (13) by at least one projection (14) extending along a height (h) in the direction of the longitudinal axis (7).