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In the scope of ultrasonic non destructive testing, the knowledge of the field of the transducer in the material to be tested is essential to locate the detected flaw and moreover to evaluate them in a precise way. We have developed a model based upon a decomposition into plane waves (angular spectrum), but while this is classically achieved for monochromatic fields, we have extended the formalism to transient domain. The first steps of the calculation are classical; from the knowledge of the transient field into a plane, a time Fourier transform permits the decompostion into monochromatic waves, then a spatial Fourier transform gives the decomposition into plane waves. Then it is possible to obtain the transient angular spectrum by mean of a change of variable and a time recomposition. The recomposition at any point of the field is then no more than a summation, with suitable delays, of all these plane waves. The advantage of this method is that the decomposition into transient plane waves (the longest operation) has to be performed only once for a given transducer, since the fit to arbitrary geometrical configurations (interface, incidence angle,...) is taken into account only at the step of the final recomposition