From https://pe2bz.philpem.me.uk/Misc/-%20Medical/-%20BrainWave/I-904-NewMindCtl/uncom.htm#USOUN

VI. ULTRASOUND AND VOICE-FM

Ultrasound is vibration of the air, a liquid, or a solid, above the
upper limit of human hearing which is roughly 15,000 Hz in adults.
Voice-FM uses a tone at or near that upper limit, and the speaker's
voice VARIES the frequency slightly. Either a "tinnitus-like sound"
or nothing is heard by the target.

Ultrasound/voice-FM can be transmitted in these ways:

- directly through the air using "air type transducers"

- directly to the brain using a modulated microwave pulse train
- through the air by piggybacking an ultrasound message on top
of commercial radio or television

The use of commercial radio or television requires that the input signal
at the transmitter be relatively powerful, since radio and TV receivers
are not designed to pass on ultrasound messages. However, the average
radio and TV receiver does not simply stop ultrasound, rather, the ability
to pass ultrasound messages "rolls off", i.e. degrades, as the frequency
is increased.

Today's radios and TVs can carry enough ultrasound messaging to be "heard"
by the human brain (though not the ear) to be effective in conveying
hypnosis. This was proven by the U.S. military forces in the Gulf War.

Ultrasound's (and voice-FM's) main advantage in mind control work is that
it can carry VERBAL hypnosis, more potent than simple biorhythm entrain-
ment.

The brain CAN "hear" and understand this "inaudible voice", while the ear
cannot. Once you can convey hypnotic suggestion which cannot be
consciously heard, you have eliminated a major barrier to the subject's
acceptance of the words being transmitted.

In previous decades, "subliminal advertising" using voice and images
at normal frequencies were "time sliced" into an apparently normal
radio or TV broadcast. This apparently did not work well, and now
voice-FM "subliminal learning tapes" commercially available have
superseded the time slice method.

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