Because modern operating systems come with utilities that measure HDD operations up to nanosecond accuracy, Ortega realized that he could use these tools to measure delays in HDD operations. The longer the delay, the louder the sound or the intense the vibration that causes it. These read-write delays allowed the researcher to reconstruct sound or vibration waves picked up by the HDD platters. A video demo is here.
"It's not accurate yet to pick up conversations," Ortega told Bleeping Computer in a private conversation. "However, there is research that can recover voice data from very low-quality signals using pattern recognition. I didn't have time to replicate the pattern-recognition portion of that research into mine. However, it's certainly applicable."
Furthermore, the researcher also used sound to attack hard drives. Ortega played a 130Hz tone to make an HDD stop responding to commands. "The Linux kernel disconnected it entirely after 120 seconds," he said. There's a video of this demo on YouTube.