adjective
a word that describes or modifies a noun or pronoun, in this case 'subaudible' describes something that is below the threshold of hearing
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The subaudible frequencies are below the threshold of human hearing.
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I can barely hear the subaudible sounds coming from the speakers.
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I think there's some subaudible stuff going on that we're not supposed to hear.
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The tension in the room was like a subaudible hum, barely noticeable but present.