If speech is used to provide warnings as well as other forms of user guidance, spoken warnings should be easily distinguishable from routine messages.
Additional Information:
For example, speech output used to identify emergency conditions might use some distinctive voice and/or preface each warning message with some other distinctive auditory alert signal. In some applications, computer- generated speech might be useful for providing a few short and simple warnings. However, if speech output is also used for other purposes, then the warning messages must be distinctive.