  The availability of digital devices operated by voice is expanding rapidly.
However, the applications of voice interfaces are still restricted. For
example, speaking in public places becomes an annoyance to the surrounding
people, and secret information should not be uttered. Environmental noise may
reduce the accuracy of speech recognition. To address these limitations, a
system to detect a user's unvoiced utterance is proposed. From internal
information observed by an ultrasonic imaging sensor attached to the underside
of the jaw, our proposed system recognizes the utterance contents without the
user's uttering voice. Our proposed deep neural network model is used to obtain
acoustic features from a sequence of ultrasound images. We confirmed that audio
signals generated by our system can control the existing smart speakers. We
also observed that a user can adjust their oral movement to learn and improve
the accuracy of their voice recognition.
