When you hear two tones, one in each ear, that are slightly different in frequency, your brain processes a beat at the difference of the frequencies. This is called a binaural beat.

Binaural beats are considered auditory illusions.

2 tones both < 1000 Hz, and no further apart than 30 Hz

One Tone for Each Ear

Right Ear 432hz - Left Ear 424hz = In addition to the two input tones, the brain now hears a non-existent third tone at 8hz (the difference between the 2 input tones)