Researchers in Germany have created a wearable computer that can authenticate users by listening to the unique sound of the wearers skull. Google Glass or VR goggles would use an integrated bone conduction speaker to blast an ultrasonic signal into the wearer’s skull. A microphone on the device would then record exactly how that signal sounds after bouncing around inside someone’s head, and the unique results could then be used to authenticate an approved user. In a controlled experiment, the devices were able to accurately recognize their assigned owners 97 percent of the time.”]
Source: https://gizmodo.com/youll-never-forget-your-password-when-its-the-sound-you-1772327137