Twenty years ago a scientist, Allan Frey, found that if a microwave carrier were to be sliced and carried audio modulation, that modulation could be heard by someone in the signals path. The thin pulses of radio carrier wave cause currents to flow through the nervous system - the result is a remote transmission; no wires or contact is needed.

"A hearing system" U.S. patent #4,877,027, 31/10/89. Wayne Brunker.
"A hearing device" U.S. patent #4,858,612, 22/8/89. Philip L.Stocklin.

Eg. The latter involves microwaves aimed at the auditory cortex. A mike turns the sounds to electrical signals which are treated so as to provide multi frequency microwaves which are applied to the brain area. Whatever sound the mike picks up (like a voice) is relayed to the target.

The first known experiment with microwaved voices was conducted by Sharp and Grove in the early 70's. However, the Defence Intelligence Agency and ARPA (The Advanced Research Projects Agency) are principally to blame for the abuse of such technologies since. eg Project Pandora etc. The CIA's Langley Research Centre as well as an army of "mad" scientists working in Energy/Defence department labs across the U.S. are also responsible.

As with the NASA Apollo program, many of those originally involved were ex Nazi or Russian Cold war scientists (even WWII Japanese) recruited, regardless of their earlier crimes, to commit more crimes, this time for the U.S.A.

It's worth noting the reported experiments carried out in bygone days included The MKULTRA (mind control)/LSD experiments, germ and nuclear fallout testing (on military and civilian personnel), electro-shock treatment on institutional victims and so on. The U.S. Energy and Justice departments are now involved in such programs so the U.S. can escape violation of international defence/agency treaties. That's also why the "D" for "defence" was dropped from DARPA.