Acoustics of clear speech: Effect of instruction
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Purpose: The current study sought to investigate how different definitions of Clear speech effect segmental and suprasegmental acoustic measures of speech. Methods: 12 neurologically healthy participants, aged 18 to 35 years served as speakers for the current study. For each speaker, 18 different sentences were selected from the Assessment of Intelligibility of Dysarthric Speech (AIDS) (Yorkston & Beukleman, 1981). Three to five occurrences of the vowels /α/, /ε/, /æ/, / &Igr;/, /[special characters omitted]/, /Λ/, /i/, and /u/ were analyzed for each speaking condition. Speakers were instructed to produce stimuli in four speaking conditions Habitual, Clear, Hearing Impaired and Overenunciate. For the Habitual condition speakers were simply asked to read the sentences. In the Clear condition speakers were instructed to "speak clearly". For the Hearing Impaired and Overenunciate conditions speakers were instructed to "say the following sentences while speaking to someone with a hearing impairment" and "overenunciate each word", respectively. Segmental acoustic measures analyzed included static measures such as vowel space area, Euclidean Distance between tense-lax vowel pairs and Intravowel Distance (ID). A dynamic measure of vowel production, termed lambda, also was obtained. Suprasegmental measures of interest included articulation rate, speaking rate, pause frequency, pause duration and sound pressure level (SPL). Data were analyzed using descriptive and parametric analyses. Results and Discussion: Results from the current study revealed that all three definitions of Clear speech differentially affected the acoustic signal relative to Habitual speech. Generally, the Overenunciate condition yielded the greatest magnitude of change in all acoustic measures followed by the conditions of Hearing Impaired and Clear. One exception was the measure of SPL, as speakers tended to produce greater SPL measures in the Hearing Impaired condition. As Clear speech is a common therapeutic technique used with patients who have dysarthria as well as with communication partners of people with a hearing impairment, the results from the current study are of clinical importance. Further research is needed to determine whether the acoustic changes reported in the current study extend to different clinical populations. Additionally, further investigation of the perceptual importance of different Clear speech definitions is needed.