Speech Disorders

Some Key Words

Principle to the discussion of disorders of speech and the phonological system is the concept of intelligibility. Intelligibility refers to how easy or difficult it is to understand a person's speech output. In addition to how accurately the speech sounds are produced by the speaker, the communicative context (topic, environment, and power differentials) and listener familiarity with the topic and the speaker also affect intelligibility. Intelligibility is likely to be adversely affected by contexts where the topic is dense, the environment distracting, and the speaker deferential. A listener familiar with the topic or the speaker's particular style can ameliorate the negative effects of the context to some extent. Improvement in intelligibility is thought to be the goal for all interventions for disorders of speech and the phonological system, but it is not always achieved solely by attention to production of specific speech sounds. A speaker can be intelligible yet still exhibit speech errors, if he or she is taught to use strategies to assist his or her listeners.

Speech is made up of speech sounds or phonemes. For the purpose of this entry, these terms can be used interchangeably, despite the nuances of their distinct definitions within the communication sciences community. Therefore, the term phonemes will be used exclusively from this point forward. The linguistic function of phonemes is to signal a change in meaning. Hence, when the phoneme /t/ is produced in the word cat it distinguishes that word from the word cap. The human vocal tract (the pharyngeal, nasal, and oral cavities) is shaped to either impede or direct airflow across the articulators (the tongue, the lips, the teeth, and the hard and soft palates) in order to create a measurable acoustic signal that differs according to the shape of the tract, the presence or absence of laryngeal voicing, and the degree of constriction through which the airflow passes.

Phoneme production varies from individual to individual. For example, the phoneme /s/ is produced in some speakers with the tongue tip pointing downward behind the bottom teeth, whereas others produce it with the tongue tip pointing up behind the top teeth. Regardless of the direction of the tongue tip, the center of the tongue is grooved and the airflow is directed into a very tight constriction in the front of the mouth. The acoustic difference between the resulting productions is not recognized in English as one that distinguishes meaning, however. This type of variation is referred to as allophonic. Each spoken phoneme, then, can truly be conceptualized as a class of acoustic signals rather than a single acceptable production. Hence, the speaker's intelligibility will be affected only if the physical production of the phoneme varies so significantly from the typical that a phoneme is produced that is outside of the class of the target phonemes. An example will help to illustrate this concept.

...