The problem of the isentropic deceleration of a supersonic flow of gas down to subsonic velocities in a channel of variable cross-section is analysed. The velocity perturbation field for different perturbation distributions in the inlet section and different perforations of the upper wall are investigated numerically. The cumulative effect of perturbations at the point of intersection of the sonic line and the upper wall is investigated, compression or rarefaction wave nucleation near the initial point on the sonic line is followed, and other effects are studied.