Silent Sound Spread Spectrum:
This is one of the most sophisticated subliminals listed here. This type of subliminal has been first described in a patent by Oliver M. Lowery, link: US5159703A - Silent subliminal presentation system - Google Patents
In brief, it is described on the patent page as such:
A silent communications system in which nonaural carriers, in the very low or very high audio frequency range or in the adjacent ultrasonic frequency spectrum, are amplitude or frequency modulated with the desired intelligence and propagated acoustically or vibrationally, for inducement into the brain, typically through the use of loudspeakers, earphones or piezoelectric transducers.
Human beings can only hear sounds in the range of 20Hz to 20,000Hz(also known as 20KHz). In this frequency range, frequencies closer 20Hz are called low frequencies and frequencies closer to 20KHz are called high frequencies. Anything above 20KHz is called ultrasonic.
As humans grow, because of noise pollution or age, their ability to hear sounds at the high frequency decreases, only children have the ability to hear up to 20KHz, most adults can only hear up to 16KHz. So from this, anything above 16KHz can be considered inaudible for an adult.
This Silent Sound Spread Spectrum patent claims that an audio message can be either frequency modulated or amplitude modulated into an inaudible high frequency. This form of subliminal doesn’t work with MP3 and other similar encoding techniques.
A plug-in for this is available for Audacity, it is mentioned in this link: Silent Subliminals [solved?]