Threshold levels for infrasound stimuli. Panel A: Data for pure-tone infrasound stimuli. Green circles and the green semi-transparent area show the median sound pressure level at threshold in dB and the corresponding interquartile range, respectively, across all 16 listeners. Gray dotted-dashed lines indicate individual thresholds; the slightly darker and thicker lines indicate thresholds of those three listeners, who had most ear-generated distortion products (see text). Panel B: The green circles are a reproduction of the median thresholds in Panel A. The gray solid line indicates the 2nd-order regression of reference thresholds reported in Figure 10 of [5]. The boxes represent threshold values reported in [4]. Crosses indicate pure-tone thresholds published in [13]; five out of their 14 listeners also participated in the present study.