1. A method for unlocking a locked computing device using a remote computing device, comprising: at the locked computing device: receiving a device identifier over a wireless communication protocol from the remote computing device, wherein the device identifier identifies the remote computing device; determining, based on the device identifier, that the remote computing device is a trusted device; causing at least a portion of an acoustic signal to be output from the remote computing device; subsequent to the portion of the acoustic signal being output, and while at least a latter portion of the acoustic signal remains to be output and is being received: receiving, via the wireless communication protocol and from the remote computing device, a digital copy of at least a portion of the acoustic signal from the remote computing device; estimating a distance between the remote computing device and the locked device based on a delay between the acoustic signal output from the remote computing device and the digital copy of the at least a portion of the acoustic signal received, via the wireless communications protocol; and unlocking the locked computing device if the distance is within a predetermined range.