A system for active noise control, ANC, uses a secondary speaker to cancel the sound pressure at an error microphone which is place at a control position in the space. This conventional ANC creates a quiet zone, QZ, around the physical error microphone. However, the QZ becomes unstable if a small change occurs in the acoustic path from the secondary speaker to the error microphone. Recently, to create the QZ virtually and to expand it spacially, a new arrangement for ANC, called the 'virtual microphone system', has been proposed. In this study, some fundamental characteristics of the virtual microphone method are clarified by both computer simulation and experiment. The difference between the acoustic pressure at the physical error microphone and that at the virtual microphone is analyzed in a three-dimensional space, and the limiting condition to be satisfied in the system is obtained. The noise reduction level of the system is compared with that of the conventional system and it is clear that they coincide very well.