Systems and methods for authorizing a user of a portable communications device entail sampling a user utterance via both an air mic (audio mic) and a conduction mic (surface mic or bone conduction mic). The difference between these signals is unique to each user since the tissues of each user will differ with respect to audio conduction. This difference may be characterized via a transform including magnitude, phase and time delay components. If the transform for a prospective user matches a stored transform for an authorized user, then the prospective user may be granted access.