Combination tones become audible only above roughly 70 dB primary level

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 (#corroborates, via linked claim)

Another interval, f2 = 2048 and f1 = 3072 Hz (and analogous intervals), is remarkable in that, first, the difference tone f = f1 – f2 = 1024 and the combination tone f = 2f2 – f1 = 1024 are the same; thus what Koenig termed the lower beat tone (the difference tone) and the upper beat tone (the combination tone) coincide. This combination has loudness beats at 1024 Hz. If the higher frequency f1 is raised by A Hz, the difference tone should increase by A Hz, but the combination tone should decrease by A Hz. Indeed, as f1 = 3072 is lowered by hand, as is possible in the MAX patch Partials, one hears both rising and falling phantom tones (a rising combination tone and a falling difference tone) interfering with each other, causing beats when they are still relatively close in frequency. When f1 = 3073 Hz, there should be two beats per second, and that is what is heard. This is also the frequency of the waveform beats. If, on the other hand, the lower frequency is raised by A Hz, the difference tone should decrease by A Hz, but the combination tone should increase by 2 A Hz, implying a beating of between them of 3 A Hz. That is heard also. Quite an instructive example! We are feeding our ears only two pure tones, both above 2000 Hz, yet we are discussing the easily audible beating of two phantom tones, both around 1000 Hz! — Eric J. Heller, Why You Hear What You Hear- An…, 2303 (#expands, via linked claim)

Categorization

• XREF: connects to: Combination tones exist below both source tones and vary in audibility; The ear generates audible combination tones not present in the stimulus — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 1571; Combination tones arise from the hearing system’s own nonlinearity — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 430 (+3 more)

Broader Connections

• Combination tones exist below both source tones and vary in audibility #expands (0.85)
• The ear generates audible combination tones not present in the stimulus #corroborates (0.80) — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 1571
• Two inaudible high tones can produce audible low phantom tones that interfere #expands (0.80) — Eric J. Heller, Why You Hear What You Hear- An…, 2303
• Two simultaneous tones generate audible lower tones absent from the stimulus #corroborates (0.80) — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 1571
• Combination tones arise from the hearing system’s own nonlinearity #expands (0.75) — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 430