Effects of Spectral Envelope and Fundamental Frequency Shifts on the Perception of Foreign-Accented Speech

To investigate the role of spectral pattern information in the perception of foreign-accented speech, we measured the effects of spectral shifts on judgments of talker discrimination, perceived naturalness, and intelligibility when listening to Mandarin-accented English and native-accented English sentences. In separate conditions, the spectral envelope and fundamental frequency (F0) contours were shifted up or down in three steps using coordinated scale factors (multiples of 8% and 30%, respectively). Experiment 1 showed that listeners perceive spectrally shifted sentences as coming from a different talker for both native-accented and foreign-accented speech. Experiment 2 demonstrated that downward shifts applied to male talkers and the largest upward shifts applied to all talkers reduced the perceived naturalness, regardless of accent. Overall, listeners rated foreign-accented speech as sounding less natural even for unshifted speech. In Experiment 3, introducing spectral shifts further lowered the intelligibility of foreign-accented speech. When speech from the same foreign-accented talker was shifted to simulate five different talkers, increased exposure failed to produce an improvement in intelligibility scores, similar to the pattern observed when listeners actually heard five foreign-accented talkers. Intelligibility of spectrally shifted native-accented speech was near ceiling performance initially, and no further improvement or decrement was observed. These experiments suggest a mechanism that utilizes spectral envelope and F0 cues in a talker-dependent manner to support the perception of foreign-accented speech.