The ear generates audible combination tones not present in the stimulus

Evidence

When two pure tones are heard together, other tones with frequencies lower than the frequencies of either of the two pure tones themselves may be heard also. These lower tones are not acoustically present in the stimulating signal, and they occur as a result of the stimulus consisting of a “combination” of at least two pure tones; they are known as “combination tones.” … The frequency of one such combination tone that is usually quite easily perceived is the difference (higher minus the lower) between the frequencies of the two tones; this is known as the “difference tone”: where fd = frequency of the difference tone fh = frequency of the higher-frequency pure tone and fl = frequency of the lower-frequency pure tone Notice that this is the beat frequency when the frequency difference is less than approximately 12. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 1571

Categorization

• DEFINE: Introduces and names the combination tone and its most perceptible case, the difference tone.
• QUESTION: Where physically are these generated — cochlear distortion or central processing? Worth chasing for the mechanism.
• corroborated: 2 independent passages support this claim — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 1571
• XREF: connects to: Combination tones arise from the hearing system’s own nonlinearity — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 430; Combination tones become audible only above roughly 70 dB primary level; Two simultaneous tones generate audible lower tones absent from the stimulus — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 1571 (+17 more)

Broader Connections

• Two simultaneous tones generate audible lower tones absent from the stimulus #expands (0.95) — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 1571
• The ear generates phantom low tones absent from the actual signal #corroborates (0.90)
• Phantom tones are heard despite being absent from the presented sound #corroborates (0.90)
• Combination tones exist below both source tones and vary in audibility #expands (0.85)
• Cubic difference tones arise inside the ear, not in the equipment #expands (0.85) — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 3354
• Nonlinear cochlear mechanics make listeners perceive nonexistent difference tones #corroborates (0.85)
• Two frequencies can mechanically generate sum and difference frequencies in the ear #corroborates (0.80)
• The human ear itself generates sound as a byproduct of amplifying incoming signals #corroborates (0.80) — George Michelsen Foy, Zero Decibels, 329
• Combination tones can be objectively measured in the outer ear canal #expands (0.80)
• What the ear perceives can diverge from sound’s physical components. #expands (0.75) — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 1568
• Difference noise generated by ear nonlinearities can be heard before the test tone itself #expands (0.75)