Subsonic

Subsonic is an ambiguous word.

A speed lower than the speed of sound is called subsonic.
Sound that is 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 10-24,000 hertz. Researchers have found that frequences as low as one hertz (one cycle per second) have a definite effect on the inner ear, affecting its equilibrium and causing dizziness. Infrasonic sound waves in nature are produced only by earthquakes.

For an article on the emotional effects of infrasound in music see infrasound.