There is a physical lower threshold for hearing - 20 Hz.

There is a physical lower threshold for hearing - 20 Hz. Sounds above this frequency are perceived through a mechanical process in the cochlea - fluid vibrates a membrane with hair cell receptors. Sounds below 20 Hz are perceived differently, since such frequencies don't vibrate the membrane - though exactly how remains unclear.

Sample: 18 participants.

The authors analyzed hearing thresholds and electrical activity in the cochlea when exposed to low-frequency tones.

Shown: at frequencies below 20 Hz, sound perception is determined not by fluid movement (as usual), but by an electrical field generated by hair cells. Sounds below 20 Hz are perceived not through the usual mechanical process in the ear, but through a unique electrical mechanism in the same ear. To put it even more simply - we hear infrasound, but the mechanism is fundamentally different. The study is essentially pilot in nature.

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Link to an article.