SPEAKE(a)R: Turning Speakers into Microphones [pdf] (2017)

SPEAKE(a)R deals with a technique that makes speaker hardware operate like a microphone, collecting signals in a direction different from its original intent.

• This is a paper of a security/hardware research nature that utilizes speakers like input devices.
• As the title suggests, it's not just a simple idea, but proposes an attack technique that demonstrates actual operational feasibility.
• This kind of research is meaningful for examining voice security, physical side-channels, and the potential for device abuse.

It's a 2017 PDF paper shared on Hacker News, and it's fair to regard it as material that includes technical experimentation and verification.