WordNet (r) 3.0 (2006):

sound barrier
    n 1: the increase in aerodynamic drag as an airplane approaches
         the speed of sound [syn: sonic barrier, sound barrier]

Moby Thesaurus II by Grady Ward, 1.0:

19 Moby Thesaurus words for "sound barrier":
   Mach, Mach number, Mach one, Mach two, air speed, escape velocity,
   ground speed, knots, shock wave, sonic barrier, sonic boom,
   sonic speed, sonics, speed of sound, subsonic speed, subsonics,
   supersonic speed, supersonics, ultrasonics