A device for controlling tightening by ultrasound comprising an electromagnetic ultrasonic transducer suitable for being electromagnetically coupled to a tightening element to be controlled with a view to generating ultrasonic waves in said tightening element and with a view to receiving and analyzing a return signal, in order to determine a first measurement of the time of flight of generated longitudinal ultrasonic waves and a second measurement of the time of flight of generated transverse ultrasonic waves, so as to deduce therefrom a tightening stress within said tightening element, said transducer being formed from a flat coil of measurement axis parallel to the longitudinal axis of said tightening element to be controlled, able to generate pulsed eddy currents at the surface of said tightening element to be controlled when said coil is supplied with at least one exciting current pulse, said coil being integrated into a system for generating a magnetic field, said system comprising a cylindrical magnet of magnetization direction parallel to the longitudinal axis of said tightening element to be controlled and placed above said coil so that the axis of revolution of said magnet is substantially coincident with the axis of said coil, characterized in that a magnetic circuit encircles said magnet and said coil with a view to guiding the generated magnetic field so that the field lines pass through obliquely in the direction of the axis of said coil an interface plane between said transducer and the surface of said tightening element to be controlled, plumb with said coil, the interaction of said field lines and of the pulsed eddy currents being suitable for simultaneously generating the longitudinal and transverse ultrasonic waves in said tightening element to be controlled, said magnetic circuit comprising: - a conical element formed by a first ferromagnetic polar part having a first end connected to a first pole of said magnet and a free second end of narrow cross section, placed in the plane of said flat coil inside said flat coil, - an annular peripheral wall of revolution about said axis of revolution, said wall being formed by a second ferromagnetic polar part that is inclined in the direction of the axis of said flat coil from a first end placed in the plane of a second pole of said magnet to a free second end placed in the plane of said flat coil outside said flat coil, said free second ends (251, 261) of said first and second polar parts defining a gap for channeling the magnetic field located in the plane of said flat coil, and - a cylindrical upper wall of revolution about said axis of revolution, said wall being formed by a third ferromagnetic polar part connected to said second pole of said magnet and securely fastened to said first end of said second polar part. - a conical element formed by a first ferromagnetic polar part having a first end connected to a first pole of said magnet and a free second end of narrow cross section, placed in the plane of said flat coil inside said flat coil, - an annular peripheral wall of revolution about said axis of revolution, said wall being formed by a second ferromagnetic polar part that is inclined in the direction of the axis of said flat coil from a first end placed in the plane of a second pole of said magnet to a free second end placed in the plane of said flat coil outside said flat coil, said free second ends (251, 261) of said first and second polar parts defining a gap for channeling the magnetic field located in the plane of said flat coil, and - a cylindrical upper wall of revolution about said axis of revolution, said wall being formed by a third ferromagnetic polar part connected to said second pole of said magnet and securely fastened to said first end of said second polar part.