  This study investigated the effect of synthetic voice of conversational agent
trained with spontaneous speech on human interactants. Specifically, we
hypothesized that humans will exhibit more social responses when interacting
with conversational agent that has a synthetic voice built on spontaneous
speech. Typically, speech synthesizers are built on a speech corpus where voice
professionals read a set of written sentences. The synthesized speech is clear
as if a newscaster were reading a news or a voice actor were playing an anime
character. However, this is quite different from spontaneous speech we speak in
everyday conversation. Recent advances in speech synthesis enabled us to build
a speech synthesizer on a spontaneous speech corpus, and to obtain a near
conversational synthesized speech with reasonable quality. By making use of
these technology, we examined whether humans produce more social responses to a
spontaneously speaking conversational agent. We conducted a large-scale
conversation experiment with a conversational agent whose utterances were
synthesized with the model trained either with spontaneous speech or read
speech. The result showed that the subjects who interacted with the agent whose
utterances were synthesized from spontaneous speech tended to show shorter
response time and a larger number of backchannels. The result of a
questionnaire showed that subjects who interacted with the agent whose
utterances were synthesized from spontaneous speech tended to rate their
conversation with the agent as closer to a human conversation. These results
suggest that speech synthesis built on spontaneous speech is essential to
realize a conversational agent as a social actor.
