When two musical notes are played simultaneously, the human ear can perceive weak additional tones called combination tones. These subjective tones result from the nonlinearity of the inner ear and are attributed to the amplification mechanism of the cochlea. Subjective tones are perceived with different intensities by different individuals. While less perceivable, objective combination tones are also generated by some musical instruments. Because these tones are present in the air, they can be detected by sensitive microphones, measured, and recorded.
Click here for original story, Capturing and analyzing subtle combination tones produced by violins
Source: Phys.org