Back — The roof or upper part in any underground mining cavity. Backfill — Mine waste or rock used to support the roof after coal removal. Barren — Said of rock or vein material containing no minerals of value, and of strata without coal, or containing coal in seams too thin to be workable. Barricading — Enclosing part of a mine to prevent .
Portable Audio. At Sam's Club®, we have the portable audio accessories you need to listen to your favorite tunes in style. From Bluetoothenabled portable speakers to wireless surround systems, we feature large and small audio components in various price ranges to make the sound come alive in your home, office or dorm room.. Types of Portable .