A simple experimental procedure based on measurements of the acoustoelectric after‐current is described for estimating the frequency of maximum intensity of the amplified acoustic flux at various stages of its buildup. In CdS, the rapid buildup in flux leading to current saturation is followed by a slower growth which continues until the flux domain leaves the filament. The frequency of maximum intensity coincides initially with the frequency of maximum gain and decreases continuously as the domain propagates down the filament, reaching in a typical case a value ten times smaller.