Hi folks -
[mod edit]
What Cliff describes in the post above is actually called "Silent Sound," and the device he speaks of takes ordinary voice, and converts it in two ways for use as SUBLIMINAL sound:
1. The normal band of voice frequencies, typically 300 to 3,000 Hz, is raised up near, but not exceeding, the upper limit of human hearing. Silent Sound voice, thus converted, centers around frequencies between 14,500 to 16,800 Hz, making it barely understandable.
2. The normal highs and lows of a voice are done away with and it is converted into a frequency modulated signal. This makes what a listener hears sound like more or less a steady tone, with the frequency chosen by the converter manufacturer, and those "center frequencies" are chosen somewhere between 14,500 to 16,800 Hz.
There are more recent variations, but the original technique was offered by Dr. Oliver Lowery as a substitute for "subliminal sound" and according to the industry is more effective. His patent was U.S. #5,159,703.
"Silent Sound" merchants claim hypnotic influence, with the subject unable, or practically unable, to hear the hypnotist's voice is achievable with this system.
The output of a Silent Sound speech converter is just an ordinary audio signal, which can be further fed to any audio equipment, radio, TV, headphones, speakers, and the like. It finds frequent use in theft prevention in department stores.
Now there actually IS a proven successful method of transmitting voice directly into the hearing sense of a subject, which uses a pulsed microwave signal, very similar to that emitted by a radar transmitter. This is commonly called "voice to skull," abbreviated "V2K" by the U.S. Army. This method does not require any electronic device on or in the person receiving the voice.
Rather than take the time to describe the process in detail here, I offer the article from the March, 1975 journal "American Psychologist," by Dr. Don Justesen, describing the successful demonstration of this principle by Dr. Joseph Sharp:
http://www.randomcollection.info/ampsychv2s.pdf(The original was light and hard to scan - use your PDF reader's zoom feature to make small print easier to read.)
A device to transmit pulsed microwave voice to skull to someone would cost in the thousands of dollars, but is do-able by an experienced technician. No exotic technology is involved.
This technology is an outgrowth of what had been called "radar hearing," an effect first reported during World War II, in which technicians working near energized radar antennas heard a buzz from the train of evenly spaced microwave pulses being transmitted by the radar set. One microwave pulse causes a single click in the hearing sense.