  Face orientation can often indicate users' intended interaction target. In
this paper, we propose FaceOri, a novel face tracking technique based on
acoustic ranging using earphones. FaceOri can leverage the speaker on a
commodity device to emit an ultrasonic chirp, which is picked up by the set of
microphones on the user's earphone, and then processed to calculate the
distance from each microphone to the device. These measurements are used to
derive the user's face orientation and distance with respect to the device. We
conduct a ground truth comparison and user study to evaluate FaceOri's
performance. The results show that the system can determine whether the user
orients to the device at a 93.5% accuracy within a 1.5 meters range.
Furthermore, FaceOri can continuously track the user's head orientation with a
median absolute error of 10.9 mm in the distance, 3.7 degrees in yaw, and 5.8
degrees in pitch. FaceOri can allow for convenient hands-free control of
devices and produce more intelligent context-aware interaction.
