An accurate distance between two devices can be determined in continuous and secure manner using modulated audible signals containing time-based information. This calculated distance can be used to lock and unlock one of the two devices such that if one of the devices, such as a smart phone or smart watch, is beyond a pre-configured distance from the other device, such as a laptop or tablet, the other device locks and may display a message to the user. The modulated messages contain time difference data of audible signal emission and receiving times which are used by each device to calculate an accurate estimate of the distance between the two devices.