writes "A recently published app for the iOS platform uses the propagation of sound waves to measure distances of up to 25 meters in a dual device mode. The technique works through repeatedly sending a chirp signal from the master device to which the other ("reflector") device synchronizes itself and then replies in a similar fashion. A novel combination of techniques has been engineered to enhance the robustness in noisy environments, such as using an optimum-autocorrelation-signal and semi-automatic frequency calibration together with an averaging over multiple cycles. Disclaimer: I'm the developer and will be available to answer your questions.
Note: I'm resubmitting this, because the earlier version had the headline truncated. Please remove this last paragraph before publishing. Thank you!"Link to Original Source