I think the photoacoustic effect is at play here. Discovered by Alexander Graham Bell has a variety of applications. It can be used to detect trace gases in g

I think the photoacoustic effect is at play here. Discovered by Alexander Graham Bell has a variety of applications. It can be used to detect trace gases in gas mixtures at the parts-per-trillion level among other things. An optical beam chopped at an audio frequency goes through a gas cell. If it is absorbed, there's a pressure wave at the chopping frequency proportional to the absorption. If not, there isn't. Synchronous detection (e.g. lock in amplifiers) knock out any signal not at the chopping frequency. You can see even tiny signals when there is no background. Hearing aid microphones make excellent and inexpensive detectors so I think that the mics in modern phones would be comparable. Contrast this with standard methods where one passes a light beam through a cell into a detector, looking for a small change in a large signal. https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Spectroscopy/Photoacoustic_Spectroscopy https://chem.libretexts.org/Bookshelves/Physical_and_Theoret... Hats off to the Michigan team for this very clever (and unnerving) demonstration.