Sound pressure levels at threshold for pure-tone infrasound stimuli. The threshold data of the 16 listeners were separated according to the absence (blue, downward-pointing triangles) and presence (red, upward-pointing triangles) of ear-generated distortion products at any higher harmonic from f₂ to f₉ of the signal frequency f₁. Open triangles indicate the median thresholds for the two groups of listeners. Semi-transparent areas indicate the corresponding inter-quartile ranges for those with distortion products (Panel B) and those without (Panel A).