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The goal of the study described in this paper was to demonstrate the use of an impulsive source and a Fast Fourier Transform (FFT) processor in an efficient and precise measurement of the complex ground impedance. In essence, this technique measures the impulse response of a sound propagation path that includes a reflection from the ground surface of interest. By simultaneously measuring both the input and output of the system (i.e., the “direct” wave and “reflected” wave) and Fourier transforming them, the Transfer Function can be evaluated. This Transfer Function is identically equal to the complex reflection factor, from which the complex impedance can be derived. Because of the difficulties reported in the past in obtaining the phase of the ground impedance, particular attention was paid to obtaining valid complex results. [This work supported by NASA Langley Research Center.]

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This paper presents the results of an experimental study of the sound radiation of in-service power transformers in the 20-MVA to 300-MVA range. It was found that (1) for transformers in the power range of 20–30 MVA, the 120 Hz tone and for transformers with power rating above 30 MVA the 240 and 360 Hz tones control the A-weighted noise emissions; (2) changing the standard transformer noise rating procedure to include measurement of the first four tones at NEMA positions would increase the accuracy of farfield noise predictions; (3) the circumferential variation of both the tone and A-weighted levels, measured at 50 and 100 ft, approximate a Gaussian distribution with ω =4 dB for each of the first four transformer tones and ω =1.75 dB for the A-weighted level; (4) the relation between the level measured at NEMA positions, L̄N, and at distance d, L̄d, is L̄d=L̄N−20 log (d) +10 log(S) −8, where d is the distance in ft. and S is the area of the vertical tank surface in ft.2; (5) it was found that changes in load had negligible influence on noise emissions. [Work was supported by the Empire State Electric Energy Research Corp., New York, NY 10020.]