Shock Subs are specialized subs designed to absorb the shock vibrations created by a down-hole hammer and to prevent damage to the drill string and the top head drive. Floating or Cushion Subs absorb shock vibrations transmitted up through the drill string built to protect the pipe. Their construction is simpler with more vertical movement in the sub. Special ID Subs are bored to a special I.D., either smaller or larger than the standard or for special cases with no I.D. bore.