# Translations:Synthetic seismogram with multiples/6/en

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We recall that the impulse response (without multiples) is generated by a purely feedforward system (with the reflection coefficients on the feedforward loops). Here, the expression for the *Z*-transform shows that the impulse response (with multiples) is generated by a feedforward-feedback system (with the reflection coefficients on the feedforward loops and the autocorrelation coefficients on the feedback loops). Equation **22**, which is for the case of one layer, is exact. In other words, equation **22** gives the *Z*-transform of the impulse response for the dynamic model (equation 1).