Subsonic has two possible meanings:

• A speed lower than the speed of sound is called subsonic.

• Sounds that are too low-pitched to be consciously perceived by humans are called infrasound or subsonic. Subsonic sound has an effect on human hearing. The frequency range of normal human hearing is 20-24,000 Hz. Researchers have found that frequences as low as 1 Hz (one cycle per second) have a definite effect on the inner ear, affecting its equilibrium and causing dizziness.
  

For an article on the emotional effects of infrasound in music see infrasound. The speed of sound c (from Latin celeritas, velocity) varies depending on the medium through which the sound waves pass. ... A schematic representation of hearing. ... Hearing is one, the auditory, of the traditional five senses, and refers to the ability to detect sound. ... Sine waves of various frequencies; the lower waves have higher frequencies than those above. ... The hertz (symbol Hz) is the SI unit of frequency. ... A left human ear. ... Equilibrium or balance is any of a number of related phenomena in the natural and social sciences. ... Dizziness (Latin: Vertigo) is the sensation of instability. ... Infrasound is sound with a frequency too low to be detected by the human ear (less than approximately 20 hertz). ...