1. A speech recognition system comprising an acoustic detector for detecting speech utterances of a speaker; a visual detector for detecting at least one facial characteristic associated with speech utterances of the speaker; a processing arrangement connected to be responsive to the acoustic and visual detectors for deriving a signal having first and second values respectively indicative of the speaker making and not making speech utterances such that the first value is derived only in response to the acoustic detector detecting a finite, nonzero acoustic response while the visual detector detects at least one facial characteristic associated with speech utterances of the speaker; and a speech recognizer for deriving an output indicative of the speech utterances as detected only by the acoustic detector, the speech recognizer being connected to be responsive to the acoustic detector only while the signal has the first value.