The effects of a flattened fundamental frequency on intelligibility at the sentence level.

The purpose of this preliminary experiment was to evaluate the effect of a flattened fundamental frequency (F0) contour on sentence intelligibility. The perceptual dimension monotone pitch is frequently used to describe the speech of persons with dysarthria, and relatively flat F0 contours have been noted in several acoustic studies of dysarthria. To determine the independent effect of a flattened F0 contour on sentence intelligibility a resynthesis technique was used that held timing and spectral characteristics of utterances constant while allowing parametric control over successive pitch periods. Two male speakers produced low-probability utterances selected from the SPIN test, which were then resynthesized with a flattened F0 contour. Speech intelligibility was assessed using two measures: one involving word transcription and the other interval scaling. These measures were collected from 10 listeners. The results showed that both measures were significantly lower when the F0 contour was flattened, as compared with naturally varying contours. Several different explanations are proposed for this effect, which can and should be explored in greater detail using the resynthesis technique given the prominence of this characteristic in dysarthria.