In order to control sources which have to reproduce a given vibratory field, the propagation in a multimode waveguide is simulated. Helmholtz's equation describes the harmonic field. Near the sources the numerical solution is calculated by the finite element method as the expansion of the solution on the series of eigen-functions of the duct is not adequate when sources are of complicated geometry and vibratory shape and when they interact. Beyond the vicinity of the sources the wave is established on certain propagated modes. At the common boundary of the two domains we connect the numerical solution and the truncated expansion of the series of propagated modes and consequently the conditions of reflexion at the end of the duct are described in an integrodifferential form on the fictive boundary. The inverse problem is to find to what voltage the sources must be submitted in order to radiate modes of a given amplitude, this on a cross-section of the duct. The iterative method of conjugate gradient allows us to obtain the solution. A method which is almost direct i.e. that of multiplicative coefficients gives more accurate results more rapidly. To conclude on these numerical aspects, a simulation of active suppression of an acoustic multimode wave is developed. The procedure is to add to the established wave with modes of measured amplitudes an "antiphase" field in such a way that the total field is zero.
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