Impossible to hear: an inaudible conversation. (adjective)
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Use "inaudible" in a sentence
"We have a project which we call our inaudible project."
"When they stop the music and speak, it is in inaudible half-whispers: for each other alone."
"The history is roughly this; the early universe, in the first submicroseconds, was extremely [word inaudible] and all of the cosmic particles, protons, electrons, unstable nuclear particles, neutrinos and photons and background radiation were all hot and were all together."