Backmasking
The term "Backmasking" has come to mean many types of auditory subliminal messages. The original term meant the reversal of words, whether hidden by music or not, so that only the unconscious mind can perceive it.
The first musical group to experiment with this phenomenon was the Beatles. It has branched out, recently, into advertising. When one is awake, and the subliminal message, say from a radio, hits the eardrum and is processed by the brain, there is still some kind of defense against it. But, unfortunately, if the listener drifts off into dreamland, they are defenseless, and the message is hopelessly wedged in a tiny corner of your brain.
Another category under the topic of backmasking is sub-band coding. There are several techniques for backmasking Sub-band coding is the process of taking an audio sample and playing it at a low enough frequency that it is not consciously perceived. The sound is hidden when played with another at a high frequency. This enables your brain to perceive the sound, though not consciously. This technique is very widely used, in music you listen to on the radio to theme songs of television shows. It is very common in the marketing world.
Beat frequency is a technique often used by self-help tape manufacturers. It is, in a way, a form of hypnotism. More creatively put, it is when two tones "bounce" off each other, and create one frequency. According to the theory, this puts the listener in a state of heightened perception. This means that anything learned or heard at this time will leave a longer-lasting impression. This may be why your English teacher may wonder why you can't memorize your prepositions, while you might be able to recite the lyrics to Cake's "Never There" after the first time of hearing it. Does that noisy old heater in the corner of your English class actually help you learn?