1862 __ Acoustic shadows in the Civil War
‣ Comment : On May 31, 1862, Confederate forces under General Joseph Johnston attempted an attack on Union forces to the east of Richmond. The ensuing battle, known as Seven Pines (or Fair Oaks), was one of Johnstons rare offensive forays during the course of the war. More comfortable with the defensive, Johnston on this occasion concocted one of the most confusing, poorly executed tactical plans of the war. Meant to synchronize forces on three converging attack routes, Confederate Major Generals James Longstreet, Benjamin Huger and D.H. Hill got their men tangled and then bickered over who had priority on the various routes. Even after this disastrous start, the Confederates still might have prevailed but for an unusual occurrence. Johnston planned to send reinforcements under Brigadier General W.H.C. Whiting on a flank attack whenever sounds of musketry were heard from Hill's troops, two miles southeast of Johnston's headquarters. The attack, if it had occurred in a timely fashion might have created a Confederate victory - but Johnston never heard the sounds of a battle which was raging in full force. This is one of the earliest examples in the Civil War of one of a type of acoustical phenomenon that had been noted for two hundred years prior to Seven Pines and given the catchall name acoustic shadows. Though citizens of Richmond could clearly hear the battle five to ten miles to their east, the sounds of musket fire eluded Johnston's ears. Similar scenarios occurred at a number of other important Civil War battles, sometimes with dramatic effects on command decisions. The term acoustic shadow describes an event in which a person who would ordinarily hear a sound does not. As mentioned below, these events also sometimes mean that those who should not hear the sound do hear it. The three most important causes of these abnormal situations are sound absorption, wind shear and temperature gradients. Sound absorption, as the name implies, occurs when material between the source and receiver of the sound waves absorbs all or part of the energy of the waves. Wind shear, in the context of this paper, describes a situation in which the winds aloft are considerably faster than the winds near the ground. The presence of wind shear can bend (or refract) sound waves downwards or upwards, depending on whether the wave is traveling upwind or downwind. Temperature gradients in the atmosphere can cause all sorts of refraction of sound waves. Especially interesting are those cases in which a temperature inversion exists: the temperature is greater at higher altitude than at lower altitudes. In this case waves are bent back down towards the ground. When sound waves that would normally refract up in to the atmosphere are refracted back down towards the ground (either by wind shear or by a temperature inversion), they may reflect off the ground back into the air and be refracted again and again. This can create concentric zones of audibility and silence around an explosive sound. Sounds from the battle of Gettysburg that could not be heard 10 miles away were heard clearly in Pittsburgh, 150 miles away. (Charles D. Ross, “Acoustic Shadows in the Civil War”, 1998). — During the American Civil War, the phenomenon of “acoustic shadows” became a source of awe, and terror. In an era when the analysis of sound and silence often drove tactical decisions, several crucial battles may have tilted to the side that benefited from the muzzling effect of foliage, wind and other atmospheric conditions. On the April morning in 1865 of the Battle of Five Forks, known as the “Waterloo of the Confederacy,” Major General George Pickett attended a fish roast hosted by another senior officer two miles from where his troops were camped. A wooded grove bordering the improvised picnic ground so effectively masked the sound of the leaderless Confederate troops being routed that some generals later claimed they had no inkling of the massive attack, with its thousands of firing rifles, until the soldiers in blue had broken through the trees within sight of where the last bones of local shad were being sucked clean of flesh. (George Rochnik, Wonders & Mysteries of the Ear) — Part Two).
‣ Urls : http://www.acoustics.org/press/136th/ross.htm (last visited ) http://inpursuitofsilence.com/2010/03/18/wonders-mysteries-of-the-ear-part-two/ (last visited )
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