Although a system like this would seem to raise privacy concerns, Yan Ke, a Carnegie Mellon researcher who worked on developing similar technology for a project at Intel, says that acoustic matching protects privacy because the hashed files can't be reverse-engineered and give no information beyond what is already in the known fingerprint. "It just so happens that the efficient method also preserves privacy," Ke says.

Amanda Welsh, IMMI's COO, agrees: "If you're carrying our phone and you're planning a bank robbery while listening to a radio, all we know is what radio station you're listening to."

Welsh says that IMMI stores personal information about volunteers separately from data collected through the cell phones, and she takes care to ensure that volunteers understand what data is being collected. "There's a small subset of people that will not do this, no matter what we say, but it's a very small subset," she says.

While Google has been researching similar technology, that technology does not seem to be mobile and is not yet in use. (See "Googling Your TV.")

Although IMMI's system works well for ambient audio, it does not yet measure things that volunteers hear through headphones or ads that they see on the Internet. Alcorn says it would be possible to run sound from a device such as an iPod through the phone, but this isn't something currently being done. Welsh says the company's next step is to improve its ability to monitor Internet advertising and media exposure.