Some lengthening factors in English speech combine additively at most rates.

The known lengthening effects of phrase-final position and of contrastive emphasis have been predicted by Klatt to combine superadditively. In a new experiment, texts elicited at a wide range of speaking rates were measured and the separate and combined effects of these lengthening factors were found to combine approximately additively at all rates studied. The proportion of lengthening attributable to each factor was found to be relatively invariant except at the fastest speaking rates, where lengthening was eventually eliminated. The results support the interpretation of absolute speaking rate as an inessential variable for characterizing speech at a range of moderate rates.