Many people have some sort of disability which impairs their ability to communicate.
Prosthetic devices of this sort must be usable in a great variety of settings. They should enable the user to be a full participant in ordinary conversations, to lead transactional encounters and to prepare speech for more formal occasions. The extent to which this is possible depends on a number of factors, both physical and cognitive. The speech impairment may be due to a physical disability which has no effect on the person's linguistic ability, or it may be due to a cognitive, language impairment. Often, some combination of physical and cognitive disabilities is involved.