A region around a sound source (e.g. a volcanic explosion) in which none of the sound is audible. Its inner boundary is the limit of surface-propagated sound, and the outer is the distance at which sound waves refracted in the upper atmosphere reach the ground. These boundaries are very approximately 100 and 200 km, respectively, from the source, but differ considerably depending on the atmosphere's temperature (and hence density) profile. They are also often far from circular, and may be affected by local topography.
Subjects: Meteorology and Climatology.