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 In this work, we review the field perturbations in inflationary models with multi-sound speeds. To be able to do explicit calculation, we consider only two fields. We obtain the evolution equations of perturbations, then we project the field perturbations along (adiabatic) and perpendicular (entropy) to the background fields’ trajectory. Direct computation, we obtain the equation of the projections evolution, these equations cannot be written in the form of an evolution equation of a perturbed field with specified field equations, (in the evolution equation of a perturbed field, the coefficient of second order spatial derivative is proportion to square of  propagation or sound speed), therefore the common formalism in which the adiabatic and entropy perturbations are defined  and  with their evolution equations, the influence on the observational parameters is investigated,  cannot be applied to this model.

Keywords: influence on the observational parameters is investigated, cannot be applied to this model.

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