Natural fast speech is perceived as faster than linearly time-compressed speech.

Listeners compensate for variation in speaking rate: In a fast context, a given sound is interpreted as longer than in a slow context. Experimental rate manipulations have been achieved either through linear compression or by using natural fast speech. However, in natural fast speech, segments are subject to processes such as reduction or deletion. If speaking rate is then defined as the number of segments per unit time, the question arises as to what impact such processes have on listeners' normalization for speaking rate. The present study tested the effect of sentence duration and fast-speech processes on rate normalization for a German vowel duration contrast. Results showed that a naturally produced short sentence containing segmental reductions and deletions led to the most "long" vowel responses whereas the long sentence with clearly articulated segments led to the fewest. This suggests that speaking rate is not merely calculated as the number of segments realized per unit time. Rather, listeners associate properties of natural fast speech with a higher speaking rate. This contrasts with earlier results and a second experiment in which perceived speaking rate was measured in an explicit task. Models of speech comprehension are evaluated with regard to the present findings.