From AMS Glossary
A region surrounding a sound source, such as an explosion, in which the sound can be detected, usually by a human without special aids.
The concept of an audibility zone may be applied to frequencies outside the range of human hearing and to detectors far more sensitive than the human ear. The existence and geometry of audibility zones depend on temperature and wind component profiles along the the path between the source and the receiver. Audibility zones close to a source depend on the wind and temperature profiles in the boundary layer. At greater distances, temperature profiles in the stratosphere and mesosphere and the location of the tropospheric jet stream are the primary determinants of the location and extent of audibility zones.