Unauthorized use of a secured device can be prevented using customized hardware that only permits operation of the secured device while an enable signal is being generated, and prevents its operation at all other times. The enable signal is only generated when variations in incoming ultrasound waves indicate that a specific ultrasound activation signal has arrived. This ultrasound activation signal is generated by an activation unit that is acoustically coupled into an ultrasound conductive body (e.g., a person's body). Because the ultrasound activation signal cannot cross air gaps, the ultrasound activation signal will only be able to reach its destination when both the activation unit and the secured device maintain acoustic contact with the ultrasound conductive body. In addition, the signal that is actually received by the secured device must be the same specific ultrasound activation signal that the secured device is expecting to receive.