Sound modules are often rack-mountable, but might also have a table-top form factor, particularly when the intended user is a DJ or record producer. The height of a sound module is often described in rack units ("U") or unit. Small sound modules are mostly 1U in height, the larger models a multiplication e.g. 2U or 3U. Despite their name, most sound modules do not produce any audible sound until their output is plugged into a keyboard amplifier or a PA system.