Crossword Genius

Sound barrier (4)

Ross

I believe the answer is:

boom

This clue is a double definition.

'sound' is the first definition.
(I know that boom is a more specific form of the action sound)

'barrier' is the second definition.
(I know that boom is a type of barrier)

(Other definitions for boom that I've seen before include "Big bang sound" , "When business is great, opposite to bust" , "Supersonic wave noise" , "The noise of increased business activity?" , "Horizontal spar on board" .)

I've seen this clue in The Globe and Mail and The Telegraph.
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