For language learners it can be very difficult to speak with- out a non-native accent. This is due to the phenomenon called language transfer. At times, this can decrease the intelligibility of the speech and make it difficult to convey the correct message to the listener. Thus, many learners have a desire to speak more naturally in order not to be misjudged due to their pronuncia- tion. For this, learners should be provided with some knowledge on what kinds of accents cause greater loss in naturalness when native speakers hear learners. In this paper, word utterances with various kinds and degrees of foreign accentedness were synthesized using a technique to morph the prosodic aspect of an utterance. These variously accented utterances were pre- sented to native speakers, who were asked to judge the natural- ness. This paper describes what degrees of prosodic morphing from native Japanese to American Japanese, Chinese Japanese, and Korean Japanese do and do not affect the naturalness per- ceived by Japanese and what kinds of words produced the least loss in naturalness for the non-native speakers.

Index Terms: Foreign language learning, foreign accent, Japanese, naturalness, acoustic morphing, listening test