Near field transient radiation patterns for circular pistons

The exact impulse response of field parameters for any field point on or off axis for the case where a circular disc radiator face is subjected to a displacement step corresponding to a velocity impulse is reviewed. By convolution, the transient field pattern for any arbitrary motion of the disc can be obtained. The exact response for a half-sine monopulse is computed. An approximate representation of the transient pressure response to the velocity impulse input at the disc is derived, and it is shown to correspond to the replica pulses described previously. The regions of validity of the approximation are quite limited and the replica pulses are displaced in time from the positions formerly attributed to them. The displaced replica approximation is applied to an examination of the structure of the near field for continuous sinusoidal excitation and a plot of positions of extrema is produced. It is shown that this approximation gives good agreement with the exact values and is superior to the previous published approach in this regard. For short sinusoidal pulses the effect of pulse length on the field pattern, and of field point on the time history of a transient wave are shown. When the excitation is a short sinusoidal pulse the effect of the pulse length and field point position on the field pattern and wave shape are demonstrated.