The study of speech naturalness in communication disorders: A systematic review of the literature.

The concept of speech naturalness is used widely in clinic and research applications. Unfortunately, the lack of consistency in research methods means that comparing findings between studies is difficult at best. In order to better understand the state of research on speech naturalness in communication disorders and quantify these impressions, this study looks at publications from the last 18 years in a systematic manner. A literature search for the exact phrase "speech naturalness" of the PubMed/MEDLINE, EBSCO, and ASHAWire databases was conducted. Articles included in the review were studies of communication and communication disorders published between 1990 and the end of 2014, in English, and in a peer-reviewed journal. 63 articles were selected and coded using a coding sheet adapted from a prior systematic review on intelligibility and cleft palate. Speech naturalness is an object of study in many subfields of communication disorders. Several concerns were raised as a result of the review, including the reliability and validity of measures, inadequate definitions of terminology, lack of detail in method descriptions, and the need to address relationships between naturalness and other variables included in the studies. Future studies should more carefully report methods and operational definitions used and more studies examining the relationship between naturalness and other speech variables in a variety of communication disorders are greatly needed.