Farfield Prediction of Time Dependent Signals

Abstract

It has been previously shown [Horace G. Ferris, “Computation of Farfield Radiation Patterns by Use of a General Integral Solution to the Time Dependent Scalar Wave Equation,” J. Acoust. Soc. Amer. 41, 394–400 (1967)] that the formulation of the farfield pressure in terms of an integral formula proceeded from the time-dependent scalar wave equation and thus had no single-frequency assumption. Thus, the farfield prediction theory should be valid for broad-band, as well as single-frequency signals. Measurements made by Hughes Aircraft Company and others have demonstrated the validity of the prediction theory, as applied to single-frequency steady-state signals. It has remained unverified, however, in the realm of time-dependent broadband signals. Measurements have been made in the Hughes Test Tank driving a narrow-bandwidth transducer with white noise. The pulse shape, as measured in the farfield has been accurately reproduced from measurements made over a small surface in the nearfield of the source. No attempts were made to predict the magnitude since such a small portion of the nearfield was covered. This suggests that the theory can be applied to the prediction of broad-band signals from measurements made in the nearfield.