Glowworm-Attack
www.nassiben.com
In this paper, we identify a new class of optical TEMPEST attacks: recovering sound by analyzing optical emanations from a device’s power indicator LED. We analyze the response of the power indicator LED of various devices to sound and show that there is an optical correlation between the sound that is played by connected speakers and the intensity of their power indicator LED due to the facts that: (1) the power indicator LED of various devices is connected directly to the power line

Some nice research from Ben-Gurion University of the Negev.

Basically 50% of devices they tested had failed to provide enough capacitance to integrate out drive power fluctuations becoming visible via power LEDs.