It took a while for the computer industry to realize that the emanations from most computer terminals and screens radiated fields strong enough that a sensitive receiver could decipher the screen contents. Although intuitive to someone with a background in radio transmission, many computer geeks simply didn't get it. To help them understand the nature of the threat, some researchers have developed files that create patterns of emanations from monitors that can be decoded by an AM shortwave receiver as music. Although spreadsheets may not be pretty to listen to, it sinks the message home. Emanations are a threat to security.
This capability, first discussed publicly by authors such as Wim van Eck, was exposed to the world as a technical curiosity (much to the chagrin of agencies that may have secretly used the techniques to eavesdrop or gather intelligence).
It is a particularly nasty effect from a security point of view because whatever is transmitted is whatever is on the screen, before any encryption systems are employed at the transmission side or after the text is decoded at the receive side.
The code word used to describe the U.S. government countermeasures to such risks is TEMPEST. Tempest is supposedly an acronym, but so many reliable sources report it to mean so many things that most users just leave it as TEMPEST.