What is "sub-bass"

Sub-bass \Sub"-bass`\, n. (Mus.) The deepest pedal stop, or the lowest tones of an organ; the fundamental or ground bass. [Written also sub-base.]
--Ayliffe.

n. (context music English) audible sounds below 60 Hz and extending downward to the lowest frequency humans can hear, typically 20 Hz.

Sub-bass sounds are those approximately below 60 Hz and extending downward to include the lowest frequency humans can hear, typically 20 Hz. In this range, human hearing is not very sensitive, so sounds in this range tend to be felt more than heard. Sound systems often feature one or more subwoofer loudspeakers that are dedicated solely to amplifying sounds in the sub-bass range. Sounds below sub-bass are called infrasound.