Acoustic near field effects in distribution transformer noise measurement

This paper examines the near field effect, including the active and reactive acoustic field around a liquid-filled distribution transformer with heat-dissipating corrugated fins on the tank walls. The analysis was carried out on the basis of measurements of sound pressure and sound intensity levels along with acoustic maps including the phase correlation of acoustic waves. The acoustic field around the transformer was evaluated in the measurement planes showing normal direction radiation of up to 1,0 m distance. Discrete point measurements made with the PU (pressure - velocity) probe was used to obtain parameters such as particle velocity, active and reactive sound intensity as well as the transfer function associated with the reference microphone. The last parameter, the transfer function, allows to show the phase correlation between acoustic wave propagation, which contributes to a better understanding of acoustic near-field effects where energy circulates, creating a sound field that can have overestimated sound pressure and intensity levels.