The problem of the interaction of long-wave perturbations of an external flow with a shock wave formed in supersonic flow past a wedge was considered in this paper. The formulas for the amplitudes of long-wave disturbances are obtained from the general analytical solution for the perturbation amplitudes. This data are compared with the results of numerical simulation. It is shown that pulsations of different modes can be separated from the total pulsation field of the incoming flow due to the different amplitude of the transformation of the perturbations of various type behind the shock wave. A method for such a separation is proposed and its approbation is carried out in a numerical experiment.