p300 components were used to investigate the processing of different sounds in the human brain function. Defining the scalp topography of the p300 components by means of reference-independent methods and identifying its electrical cortical generators by using the standardized low-resolution electromagnetic tomography (sloreta) are the aims of this study. Compared to the rare non-target tone, the rare target tone elicited a more prominent p300 occurring later than that of the rare non-target one. The rare non-target and rare target stimuli showed different scalp topography and cortical sources. Sloreta computation of the p300 responses yielded clear frontal region in both stimulus types. However, the rare non-target tone electrical filed had a more posterior distribution as compared to the rare target tone and its generators were localized in frontal area. The rare target tone produced p300 maximum over the medial frontal gyrus (mfg), whereas maximum of the p300 activated by the rare non-target tone was observed over the superior frontal gyrus (sfg). Differences in scalp topography and cortical sources suggest that two stimulus types reflect different neural processes. The findings on cortical generators are in line with the hypothesis that p3a reflects the automatic allocation of attention, while p3b is related to the effortful processing of task-relevant events.