A Survey of Using Vocal Prosody to Convey Emotion in Robot Speech

The use of speech for robots to communicate with their human users has been facilitated by improvements in speech synthesis technology. Now that the intelligibility of synthetic speech has advanced to the point that speech synthesizers are a widely accepted and used technology, what are other aspects of speech synthesis that can be used to improve the quality of human-robot interaction? The communication of emotions through changes in vocal prosody is one way to make synthesized speech sound more natural. This article reviews the use of vocal prosody to convey emotions between humans, the use of vocal prosody by agents and avatars to convey emotions to their human users, and previous work within the human-robot interaction (HRI) community addressing the use of vocal prosody in robot speech. The goals of this article are (1) to highlight the ability and importance of using vocal prosody to convey emotions within robot speech and (2) to identify experimental design issues when using emotional robot speech in user studies.