A report recently released by the Pentagon under a Freedom Of Information Act request shows us the way. PhysOrg reports:
"The phenomenon is tunable in that the characteristic sounds and intensities of those sounds depend on the characteristics of the RF energy as delivered," the report explains. "Because the frequency of the sound heard is dependent on the pulse characteristics of the RF energy, it seems possible that this technology could be developed to the point where words could be transmitted to be heard like the spoken word, except that it could only be heard within a person´s head. In one experiment, communication of the words from one to ten using ´speech modulated´ microwave energy was successfully demonstrated. Microphones next to the person experiencing the voice could not pick up these sounds. Additional development of this would open up a wide range of possibilities."
The report predicts that communicating at longer distances would be possible with larger equipment, while shorter range signals could be generated with portable equipment. Putting voices in people´s heads could cause what the report calls "psychologically devastating" effects.