It has been demonstrated that speech rhythm classes (e.g. stress-timed, syllable-timed) can be distinguished acoustically and perceptually on the basis of the variability of consonantal and vocalic interval durations. It has moreover been shown that even infants are able to use these cues to distinguish between languages from different rhythm classes. Here we demonstrate that the same classification is possible in the acoustic domain based simply on the durational variability of voiced and voiceless intervals in speech. The advantages of such a procedure will be discussed and we will argue that 'voice' possibly offers a more plausible cue for infants to distinguish between languages of different rhythmic class.