Silent Speech Interfaces have been proposed for communication in silent conditions or as a new means of restoring the voice of persons who have undergone a laryngectomy. To operate such a device, the user must articulate silently. Isolated word recognition tests performed with fixed and portable ultrasound based silent speech interface equipment show that systems trained on vocalized speech exhibit reduced performance when tested on silent articulation, but that training with silently articulated speech allows to recover much of this loss.