inaudible

from WordNet (r) 3.0 (2006)

inaudible
    adj 1: impossible to hear; imperceptible by the ear; "an
           inaudible conversation" [syn: {inaudible}, {unhearable}]
           [ant: {audible}, {hearable}]

from The Collaborative International Dictionary of English v.0.48

Inaudible \In*au"di*ble\, a. [L. inaudibilis; pref. in- not +
   audire to hear: cf. F. unaudible. See {In-} not, and
   {Audible}.]
   Not audible; incapable of being heard; silent. --
   {In*au"di*ble*ness}, n. -- {In*au"di*bly}, adv.
   [1913 Webster]

from Moby Thesaurus II by Grady Ward, 1.0

21 Moby Thesaurus words for "inaudible":
      echoless, hush as death, hushed, mousy, noiseless, quiescent,
      quiet, silent, soundless, still, still as death, stilly,
      subaudible, unarticulated, unhearable, unpronounced, unsounded,
      unuttered, unvocalized, unvoiced, whist

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