A method system for use of Doppler shift as a basis to detect user focus, such as to detect that a user was attracted to audio media and/or to an associated object. A portable processing device carried by the user receives audio media emitted from an audio source at a fixed location, the audio media having periodic watermarking encoded at a baseline frequency. The portable processing device detects a change in frequency of the periodic watermarking over time, such as the frequency progressing from at least being higher than the baseline frequency to being the baseline frequency for at least a predefined threshold period of time. Based on the detected change in frequency of the periodic watermarking over time, the portable device then provides a report indicating that the user was attracted to the audio media and/or to an object (e.g., a commercial object) collocated with the audio source.