It must be one of the worst ways to build a microphone imaginable, tapping into a disk drive's nanosecond head stops as it waits for the vibrations caused by sound to cease, but it has been done.
When sound meets a disk drive, the sonic vibrations cause a shaking in the disk drive and the platters vibrate. The drive's control logic then halts read/write head movements until the platter-shaking stops, wanting to avoid head crashes. Disk drive tools measure drive operations to the nanosecond, meaning that read/write head stillness can be measured; its start time, stop time and duration.
By monitoring the sequence and duration of head stops, the profile of an incoming sound wave can thus be created.
Software security consultant Alfredo Ortega discussed how this was done at the Ekoparty conference in Buenos Aires with a talk entitled "Turning hard disk drives into accidental microphones".
Alfredo Ortega's Ekoparty video teaser on the disk drive microphone
Once he recorded this head stop data on a PC, he was able to play the resulting wave through its loud speaker and so reproduce, badly, the original sound. It's not a sensitive enough measurement to reproduce human conversation, although Ortega thinks this might ultimately be possible by applying pattern recognition techniques to the low quality signal data he has recorded.
The disk drive's operation can be stopped entirely by directing the right sound waves at it and using the drive's resonance to amplify the incoming sound-induced vibrations. Ortega found a 130Hz tone played for two seconds caused a disk drive to shut itself down.
In September 2016 disk drives in a Romanian ING bank data centre shut down when gas was released with a great noisy whine in a test of an Inergen fire suppressant system. The sounds were louder than 130dB, like a fighter jet taking off with full thrust.
A Sun engineer, Brendan Gregg, also caused disk drives to delay operations by shouting at drives in a JBOD in 2008.
Brendan Gregg "Don't shout at your JBOD" YouTube video.
All this is great fun but is there a real security angle here? Sounds like there might conceivably be. ®