smartphone to surreptitiously eavesdrop on conversations in a room—not with a gadget’s microphone, but with its gyroscopes, the sensors designed measure the phone’s orientation.

“Whenever you grant anyone access to sensors on a device, you’re going to have unintended consequences,” says Dan Boneh, a computer security professor at Stanford. “In this case the unintended consequence is that they can pick up not just phone vibrations, but air vibrations.”

When the researchers tested their gyroscope snooping trick’s ability to pick up the numbers one through ten and the syllable “oh”—a simulation of what might be necessary to steal a credit card number, for instance—it could identify as many as 65 percent of digits spoken in the same room as the device by a single speaker.

Google’s Android operating system allows movements from the sensors to be read at 200 hertz, or 200 times per second. Since most human voices range from 80 to 250 hertz, the sensor can pick up a significant portion of those voices. Though the result is unintelligible to the human ear, Stanford researcher Yan Michalevsky and Rafael’s Gabi Nakibly built a custom speech recognition program designed to interpret it.