inaudible
adjective C2- Unable to be heard or not loud enough to be heard. · Dogs can hear this whistle, but for people it is inaudible.
From the library
Dantès in his cell heard the noise of preparation,—sounds that at the depth where he lay would have been inaudible to any but the ear of a prisoner, who could hear the splash of the drop of water that every hour fell from the roof of his dungeon.
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