1. A method for recognizing an unknown speaker, comprising the steps of: directing an ultrasonic signal at a face of an unknown speaker; acquiring a Doppler signal of the ultrasonic signal after reflection by the face; extracting Doppler features from the reflected Doppler signal; and classifying the Doppler features using Doppler models storing the Doppler features and identities of known speakers to recognize and identify the unknown speaker, wherein there is one model for each known speaker.