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The acoustic emission signals produced by discharges of a point-plane electrode on the surface (surface discharge) of different oil-impregnated aramid pressboards are detected by a 150 kHz acoustic emission sensor mounted on the outside of the experimental tank sample. Numbers, maximum amplitudes and average amplitudes of acoustic bursts are measured via a digital signal processor device and evaluated. The effect of transition mechanism from spark to pseudoglow discharge on acoustic emission signals at different voltages is examined. The tracking patterns on the surface of four different aramid pressboard insulations are compared together by using Matlab image processing program. By experimental results, tracking patterns on the surface of the bulk impregnated pressboard insulation have a significant effect on the acoustic emission signals. Acoustic emission signals amplitudes decrease dramatically on damaged pressboards and cause so called silent PDs. [ABSTRACT FROM AUTHOR]