starzia writes "Researchers at Northwestern University and the University of Michigan have developed a technique which aims to extend the reach of mobile phone location tracking. Their free iPhone app, Batphone, extracts a location 'fingerprint' from a short recording of ambient sound. This software-only approach allows the device to determine its location with high accuracy using its built-in microphone. Unlike prior indoor tracking techniques, Batphone does not rely on the presence of Wi-Fi access points to serve as landmarks, although these can be used to assist the system when available. They also posted a web game which allows you to test your own ability to recognize rooms by listening. Technical details are in a paper which was presented at the MobiSys conference on Thursday. This is from the same people who brought you laptop sonar."