Sub-bass is a term used to describe audible sounds below 90Hz - sound in the range below this (under 16-17Hz) would be termed infrasound. Sound systems often feature a subwoofer that is dedicated solely to amplifying sounds in the lower bass and sub-bass range.
When reproduced at sufficient volume, the listener will feel sub-bass more than it is heard - for this reason, it is popular in much dance music, where the sub-bass will often accompany the bass drum, as well as in dub reggae, and influenced genres such as dubstep - where the whole bass-line is often reproduced in sub-bass frequencies. Much experimental music uses sub-bass - in particular drone music, where the majority of the sound can often be in the sub-bass range. Metal bands such as Cephalic Carnage and The Devil Wears Prada have used sub-bass drops in their songs.
In vocal music, sub-bass is an informal term for a male singer who typically sings in a range below that of bass. Sub-bass singers are most often found in southern gospel groups or a cappella contemporary Christian groups. These singers often sing below the F clef and sometimes employ vocal fry to achieve even lower sounds.
Sub-bass is also a loose translation of basso profondo.