
When two different tones of specific frequencies are played through headphones, the brain can become confused and produce its own, imagined tone - a 3D audio hallucination heard only within the head of the listener. The frequencies that produce this phenomenon are known as Binaural Beats.
For example, if a frequency of 100 Hz is played in one ear and a frequency of 110 Hz is played in the other ear, the result is a binaural beat of 10 Hz, created by the brain.
What is actually happening is that the brain is not used to hearing frequencies in each ear so close together and with such intensity as these sounds do not occur in nature and there is no mechanism in our brains to understand them. Instead, the superior olivary nucleus, the area of the brain which controls aspects of 3D sound perception, bridges the difference between the varying frequencies in Binaural Beats with a common 'third tone' in an attempt to normalize this audio into something we can understand.
What is really weird is tat each person hears the third tone differently. It was proved that people with Parkinson's disease can't hear it at all, while women will hear different tones as they move through their menstrual cycle.