A good knowledge of the velocity distribution on a vibrating surface permits computing the near field from this surface correctly. A method is described which determines the amplitudes and phases of the displacement at each point on a source (here a loudspeaker diaphragm) with harmonic excitation. At first the vibrations of the shell are visualized and the amplitudes measured by means of time-averaged optical holography interferometry. Once the amplitude distribution of the displacement of the surface has been thus obtained, a capacitive transducer is used in such a way that the relative phases of the displacement from one point to another can be determined.
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