Echoing Questions: Acoustic Shadows and the Battle of Gettysburg 

During the Civil War, commanders often used the sounds of skirmishes to judge the course of the fighting and to make decisions about troop maneuvers in individual battles. But there were times when something odd occurred: people close to the hostilities heard almost nothing at all, while others many miles away did. Called an "acoustic shadow," this phenomenon was reported during the bloody clash at Gettysburg from July 1 through 3, 1863, when people more than 150 miles distant in Pittsburgh reported hearing the thunder of cannons, while farm families a few miles down the road heard absolutely nothing.
Recently, a Johns Hopkins physicist and an alumnus who is a tour guide at the Gettysburg National Military Park discussed acoustic shadows: the science behind why they happen, and what actually did happen -- or didn't -- in terms of sound during that one historic battle.