There is also interaction of different notes somewhere in the processing in the brain. On speakers play a sine wave in the left channel and a sine wave of a sl

There is also interaction of different notes somewhere in the processing in the brain. On speakers play a sine wave in the left channel and a sine wave of a slightly different frequency in the right channel. You'll hear a beat frequency due to the actual physical interaction of the two waves in the air. Then do the same thing except using headphones with good isolation so each ear only hears one of the sine waves. You still hear a beat frequency even though the two waves are no longer interacting in their air.