The purpose of this study was twofold: to determine through psychophysical comparison of scaling data whether speech naturalness is a prothetic or a metathetic continuum, and to examine the relationship between selected acoustic characteristics of the speech of nonstutterers and treated stutterers and listeners' judgments of their speech naturalness. Comparison of magnitude estimation and interval scaling data indicated that speech naturalness behaves like a metathetic continuum, suggesting that either scaling procedure is valid for the quantification of this dimension. The speech of the nonstutterers was judged more natural than the speech of the treated stutterers, and a global voice onset time (VOT) measure (averaged across places of articulation) and a sentence duration measure were found to be the acoustic parameters most highly correlated with and predictive of speech naturalness. These results suggest the possibility that stuttering treatments that employ strategies like gentle voicing onset and prolonged speech may result in somewhat slower posttherapy speech patterns characterized by prolonged VOTs that could influence listeners to judge the speech as more unnatural than the speech of nonstutterers.

The appropriateness of direct magnitude estimation and interval scaling for assessing stuttering severity was investigated by determining whether the continuum of the stutterers' judged severity was prothetic or metathetic. As operationally defined by Stevens, prothetic continua show a curvilinear relation between magnitude estimates and interval scale values of the same set of stimuli, whereas metathetic continua show a linear relation between these scale values. The stuttering severity of 20 stutterers was scaled by three groups of 15 listeners who used interval scaling, direct magnitude estimation with standard/modulus, and direct magnitude estimation without standard/modulus. The results indicated that the two sets of direct magnitude estimation scale values were related to the interval scale values in the curvilinear fashion that is typical of prothetic continua. These findings suggest that direct magnitude estimation is preferable to interval scaling for measuring stuttering severity.

The speech of 14 stutterers was analyzed prior to and at the termination of a 5-week stuttering therapy program to examine the relationship between nine selected acoustic variables and stuttering frequency. Group analyses indicated that pre- to post- therapy changes in stuttering frequency were accompanied by mean changes in five of the nine acoustic variables, a finding which is consistent with previous literature. Correlational analyses indicated that only silence in the voiced stop consonant intervocalic interval (IVI) was significantly correlated with stuttering frequency prior to therapy (i.e., lower stuttering frequency values were associated with shorter durations of silence during the IVI). Furthermore, the degree of reduction in silence was positively correlated with the magnitude of reduction in stuttering frequency due to therapy. These findings suggest that silence in the IVI may reflect the operational status of some mechanism which may underlie disfluent speech.

The fundamental frequency (f Ø ) of the first five periods in the acoustic wave form of vowels following stop consonant productions and the f Ø of a period approximately 100 ms into the vowel were analyzed in the repeated fluent utterances of 10 nonstutterers and 10 stutterers both pre- and posttherapy. Group data indicate that the nonstutterers and stutterers showed similar f Ø diminution patterns in vowels immediately following stop consonants. Additionally, the stutterers were not significantly different from the nonstutterers in their ability to achieve a stable f Ø over repeated utterances. These results are discussed with respect to a previous analysis of the present data (Sacco & Metz, 1986) in which it was found that stutterers were significantly more variable than nonstutterers in their ability to achieve a stable f Ø over repeated utterances. It is suggested that stutterings in the immediate vicinity of otherwise fluently produced words may influence certain production characteristics of those words.

Stuttering frequency and four aerodynamic measures of articulatory events within perceptually fluent voiced and voiceless intervocalic intervals were obtained from 15 stutterers prior to and at the termination of a concentrated program of stuttering therapy. Three of the four aerodynamic variables showed group changes over the course of therapy concomitant with improvements in fluency. Additional correlational analyses suggested that one of the aerodynamic variables was specifically related to stuttering frequency. It is argued that both fluency enhancing and ancillary components of therapy operate to influence the intervocalic interval.