It is recommended that each device should speak only one language. Echo of confirmation on all functions and figures is required, including the on function. Digitised speech is recommended. The specification for speech is 64 Kbps. It is advisable to be careful about regional accents.
Volume control is needed.
The same knob or slide used for volume control can however be used for switching off the speech. Good contrast is required between text or figures and buttons. It is recommended to have a combination of a first letter in upper case, with following letters in lower case. Text is required to be as large as possible. Abbreviations can be used as long as they are understandable and explained in the instructions.