Impossible to hear; imperceptible by the ear:
— An inaudible conversation.

synonym: unhearable.

Roget 403: silent; still, stilly; noiseless, soundless; hushed etc. v.; mute etc. 581.    soft, solemn, awful, deathlike, silent as the grave; inaudible etc. (faint) 405.    ... show more

Roget 405: inaudible; scarcely audible, just audible; low, dull; stifled, muffled; hoarse, husky; gentle, soft, faint; floating; purling, flowing etc. v.; whispered etc. v.; liquid; soothing; dulcet etc. (melodious) 413; susurrant^†, susurrous^†.    ... show more

Roget 419: deaf, earless^†, surd; hard of hearing, dull of hearing; deaf-mute, stunned, deafened; stone deaf; deaf as a post, deaf as an adder, deaf as a beetle, deaf as a trunkmaker^†.    inaudible, out of hearing.    ... show more

Roget 581: aphonous^†, dumb, mute; deafmute, deaf and dumb; mum; tongue-tied; breathless, tongueless, voiceless, speechless, wordless; mute as a fish, mute as a stockfish^†, mute as a mackerel; silent etc. (taciturn) 585; muzzled; inarticulate, inaudible.    croaking, raucous, hoarse, husky, dry, hollow, sepulchral, hoarse as a raven; rough.    ... show more

Dutch: onhoorbaar
Polish: cichy