Upward masking makes intermodulation difference tones more audible than harmonics

Evidence

Engineers have invented several ways to quantify the magnitude of the non-linearity by measuring the distortion products and expressing them as a percentage of the original input signal, or of that signal plus distortion and noise. If the input signal is a pure tone, the distortion products are harmonic overtones of that signal: harmonic distortion. The problem with that method is that as the fundamental frequency gets higher, the higher harmonics move beyond the hearing range so it ceases to be a relevant metric. Combining two closely spaced tones and scanning them to high frequencies is more useful. Each of the tones will generate a set of harmonic distortions, but they will also interact with each other in the non-linearity and create combination tones—intermodulation distortions. These are sum and difference multiples of the two input signals and their harmonics that extend upwards and downwards in frequency. Because masking is much more effective in the upward frequency direction (see Figure 9.14), it is the difference frequencies that end up being more audible and, because they are inherently unmusical, more annoying. — Toole, Floyd E, Sound reproduction- the acoustics and…, 989

Categorization

• XREF: The upward masking asymmetry (Figure 9.14) is a core psychoacoustics principle; connects to how spectral masking shapes perceptual salience, relevant to any masking-based audio model. — connects to: Masking spreads from a noise band to tones further away as masker level rises; Cubic difference tones arise inside the ear, not in the equipment — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 3354; Difference noise generated by ear nonlinearities can be heard before the test tone itself (+7 more)
• DEFINE: Distinguishes harmonic distortion from intermodulation distortion, and explains why the two-tone test is more relevant at high frequencies.
• corroborated: 2 independent passages support this claim — Toole, Floyd E, Sound reproduction- the acoustics and…, 989

Broader Connections

• Increasing stimulus level broadens auditory filters and causes upward spread of masking #expands (0.90)
• Simultaneous tones become harder to perceive due to masking effects #expands (0.85) — David Howard, Jamie Angus, coustics and Psychoacoustics, 1628
• Loud speech can mask its own high-frequency components via upward spread of masking #exemplifies (0.80) — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 2480