NORFOLK, VIRGINIA--Bad acoustics may have shaped the outcome of key battles during the U.S. Civil War, according to research presented today at a meeting of the Acoustical Society of America. The culprits--winds and weather systems--appear in some cases to have swept away artillery sounds and left commanders unaware that the fighting had begun.
Charles Ross, a physicist at Longwood College in Farmville, Virginia, was researching a book on the Civil War when he noticed something odd: 10 battles in which generals seemed inexplicably deaf to action at the front. During the 31 May 1862 battle of Seven Pines, Virginia, for instance, Confederate General Joseph Johnston sat in silence near Fair Oaks, 3 kilometers behind the front line. Fighting there was in full swing, but Johnston, hearing nothing, refused to send in key reinforcements until it was too late. That battle ended in a draw, but "it should have been a Confederate victory," says Ross. Even stranger, sounds of gun and cannon fire that day easily reached Richmond, some 16 kilometers behind the front line.
Such spots of sound and silence can occur, Ross points out, in a temperature inversion, where gradients reverse their normal pattern and the air is warmer at higher altitudes than near the ground. Because sound travels faster in warmer air, the top part of a wave heading upward on a slant moves faster than the bottom part, flipping the wave down toward Earth. Under these conditions, sound waves can bounce between Earth and the atmosphere for miles, delivering sound to some regions but leaving others in an "acoustic shadow."
Ross suspects that an inversion hung over the battlefield at Seven Pines. Digging through soldiers' diaries, he found accounts of a thunderstorm the night before battle and low cloud cover on the day of--perfect conditions for a temperature inversion, he says. In other battles, Ross suspects that strong winds tipped sound waves on their heels, sending them skyward so they couldn't be heard.
"It's completely plausible," says David Havelock, a physicist with the National Research Council in Ottawa, Canada, an expert on atmospheric acoustics. During some former U.S. nuclear weapons tests, temperature inversions occasionally created periodic pockets where the blast was quieted, he points out. If General Johnston had kept a physicist in his ranks, he might not have trusted his ears to verify the course of battle.