US Patent 7623823 Detecting and measuring exposure to media content items

User exposure to media items is detected and measured. A mobile client device carried by a user digitally samples the audio environment of the user on a regular basis. These samples are transformed into a stream of data signatures and compared with reference media items to detect user exposure to the reference items. Purchase behavior following exposure to selected media content items can be detected and tracked as well, so as to gauge effectiveness of media items such as advertisements.