1. A method of operating a man-machine interface unit, comprising: classifying at least one utterance of a speaker to be of a first type or of a second type, wherein if said utterance is classified to be of said first type, said utterance belongs to a known speaker of a speaker data base, and if said utterance is classified to be of said second type, said utterance belongs to an unknown speaker that is not included in said speaker data base; storing a set of utterances of said second type; clustering said set of utterances into clusters, wherein each cluster comprises utterances having similar features; automatically adding a new speaker to said speaker data base based on utterances of one of said clusters.