Definition of the sound barrier noun from the Oxford Advanced Learner's Dictionary

 

the sound barrier

 noun
noun
BrE
 
; NAmE
 
[singular]
 
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the point at which an aircraft’s speed is the same as the speed of sound, causing reduced control, a very loud noise (called a sonic boom) and various other effects to break the sound barrier (= to travel faster than the speed of sound)
See the Oxford Advanced American Dictionary entry: the sound barrier