Speech synthesis in background noise: effects of message formulation and visual information on the intelligibility of American English DECTalk

The purpose of the current research was to investigate the intelligibility of synthesized speech in noise, when listeners are able to watch an individual using augmentative and alternative communication (AAC) formulate messages on-line and when they are listening to a speaker without any visual information. A total of 80 participants were randomly assigned to four groups, with 20 participants in each group. Each group listened to sentences delivered using a different message formulation strategy: prestored; audibly formulated (messages are formulated on-line and the listener is able to hear the formulation as the message is being encoded); audibly formulated with no repeat (the full sentence at the end is not repeated); and quietly formulated (the message is formulated on-line, but the listener is not able to hear the system feedback throughout the formulation). The speaker for this study was a 35-year-old woman with cerebral palsy who used a VOCA with DECTalk (Beautiful Betty, American English) to communicate. Half of the sentences were presented in an auditory-only condition and half were presented in an auditory-visual condition. The dependent variable was intelligibility, as measured by the percentage of words correctly transcribed by each listener. The overall intelligibility of the sentences in the Audibly Formulated with No Repeat group was statistically significantly lower than in each of the other message formulation type groups. Visual information did not have an effect on intelligibility for this speaker. Clinical implications, limitations, and directions for future research and development are discussed.