anacoustic

[ an-uh-koo-stik ]
/ ˌæn əˈku stɪk /

adjective

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

British Dictionary definitions for anacoustic

anacoustic
/ (ˌænəˈkuːstɪk) /

adjective

unable to support the propagation of sound; soundless