You searched for: “anacoustic zone
anacoustic zone; zone of silence (s) (noun); anacoustic zones; zones of silence (pl)
An area in outer space where sound cannot be transmitted: The anacoustic zone is said to be the upper portion of the earth's atmosphere starting at a hundred miles (160 kilometers) and on into interplanetary space, where sound cannot be projected because gas molecules are too far apart to serve as a transferring medium.

The anacoustic zone is also known as the "zone of silence".