In physics and systems theory, the superposition principle, also known as superposition property, states that, for all linear systems, the net response at a given place and time caused by two or more stimuli is the sum of the responses which would have been caused by each stimulus individually. (Wikipedia)
"The LAW is strictly true only when the amplitudes are exceedingly small. When the disturbance of the air by a sounding body is so violent that the law no longer holds good, secondary waves are formed, which correspond to the harmonic tones of the sounding body." See Law of Cycles
"Resultant tones are of two kinds; the one class corresponding to rates of vibration equal to the difference of the rates of the two primaries; the other class corresponding to the rates of vibration equal to the sum of the two primaries. The former are called DIFFERENCE TONES; the latter SUMMATION TONES." [Tyndall, John; Sound.] See Law of Cycles
See Additive and Subtractive Frequency and Amplitude Modulation - AM and FM.