Researchers have managed to cheaply harvest 4,096-bit encryption keys from distances of around 10 metres (33 feet) using a microphone. As a computer’s processor churns through the encryption calculations, the machine emits a high-frequency “coil whine” from the changing electrical current flowing through its components.
http://www.theregister.co.uk/2016/06/04/sidechannel_encryption_theft/
http://cacm.acm.org/magazines/2016/6/202646-physical-key-extraction-attacks-on-pcs/fulltext