An “acoustic shadow” is when the sounds of an event—here, a battle—cannot be heard by people nearby—say, in the neighboring valley or a parallel city street—but those same sounds can plainly be heard over much larger distances. This effect is caused by “a unique combination of factors such as wind, weather, temperature, land topography, forest or other vegetation, and elevation,” (via BLDGBLOG: Events in the Landscape and their Acoustic Shadows
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