The acoustic source we study is a vortex whistle composed of three parts of geometry and dimension similar to that of a bottle. The device consists of a cylindrical cavity with a circular base, a single fluid inlet tangential to the base of the cylinder and a cylindrical fluid outlet downstream which is a cylinder with a radius smaller than that of the cavity. The fluid (here air) entering the cavity generates a circular motion in the cavity and in the downstream cylinder. This device produces a sound with a dominant frequency. The frequency of the sound is controlled by the fluid flow rate. The characterization of the fluid vortex is performed in air by stereo-PIV (Particle Image Velocimetry), a laser optical diagnostics, resolved temporally and spatially, combined with acoustic measurements by microphone. In the exit section of the whistle, the center of the vortex circulates with a precession motion. From a decomposition into eigenmodes of the velocity field, the contributions to this motion are specified. The directivity diagram is established by calculating the Rayleigh integral based on the measured velocity fluctuations. The coupling between the acoustic field and the flow dynamics is thus highlighted and analyzed..