of or relating to the upper portion of the earth's atmosphere and to interplanetary space, where sound cannot be transmitted because molecules are too far apart to serve as a transmitting medium.
Origin of anacoustic
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
unable to support the propagation of sound; soundless
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012