The effect of the medium immediately surrounding the shot on the properties of the primary compressional waves has been studied by means of two-dimensional models. Circular disks of Plexiglas and Styrofoam of various diameters were inserted in large sheets of aluminum and of Plexiglas, respectively. Charges were fired in the center of the disk. An approximate theory for radiation from a simple harmonic line source of P waves on the axis of a solid circular cylinder embedded in an infinite homogeneous solid medium has been developed. The transfer function of the disk has been calculated from the observed spectra in order to compare it with the frequency response calculated from theory. The results show that for these cases of a low-velocity disk a peak frequency exists which decreases with increasing diameter. The maximum peak frequency is that for a shot in an infinite sheet of the high-velocity material, and the minimum peak frequency is that for a shot in a sheet of the low-velocity disk material. This minimum peak frequency is approached rapidly as the disk diameter increases.The geometric effect of radiation from the circular inhomogeneity is important only over a narrow range of cavity diameters for which the resonance associated with the cavity happens to fall near the peak of the spectrum of the explosion-generated wave.