1. Apparatus for detecting whether a portion of an audio signal generated over successive time periods contains speech to be recognized, said apparatus comprising: speech detection means for comparing the amplitude of the audio signal during successive time periods with one or more amplitude thresholds, and for generating, in response to said comparisons, an indication of whether or not a given portion of said audio signal contains speech to be recognized; means for deriving a background amplitude level from the amplitude of said audio signal for one or more time periods in which the signal does not contain speech to be recognized, which level indicates the amplitude of the audio signal when it does not represent speech to be recognized; means for deriving a measure of the spread of the distribution of the background amplitude level; and means for altering, for purposes of the comparisons of the speech detection means, the relative magnitude of the audio signal amplitudes and the amplitude thresholds as a function of the background amplitude level and spread.