The new frequency produced is the difference between the two original frequencies, and is only "heard" when the listener clearly distinguishes each sound between each ear. (headphones work best for this effect, stereo output is required.)

This third frequency is referred to as a "Binaural Beat" - and allows the listener to perceive frequencies lower than they can detect through normal hearing.

While this also has it's applications outside of the self-help/hypnosis realm, it is significant because it allows us to create frequencies that match those our brains already use.

Our human ears can only detect frequencies between 20 and 20,000 Hz, while our brain normally operates between 1 and 70 Hz or so. That means it's not possible to use traditional audio applications to get anywhere in the lower ranges.

• Gamma waves are 40 Hz or higher.
• Beta waves are between 13 and 40 Hz, and are usually indicative of attention, deep thinking and/or concentration.
• Alpha waves are between 7 and 13 Hz, indicating a relaxed, but wakeful state.
• Theta waves are between 4 and 7 Hz, and normally happen during dreams.
• Delta waves are those lower than 4 Hz, and usually indicate deep sleep (without dreaming).

Using binaural beats, it's possible to match these lower brain wave frequencies by combining the right sounds. For example, a tone at 350 Hz in one ear and another at 360 Hz in the other will make a binaural beat at 10 Hz, which corresponds to the brains Alpha waves.

Theoretically, when listening to binaural beats the brain will lean towards the same frequencies it is "hearing", which in turn can help to produce the desired state (Alpha waves for most self-hypnosis techniques, Beta waves for audio studying aids).