Acoustical Society of America
136th Meeting Lay Language Papers

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Acoustic Shadows in the Civil War

Charles D. Ross>
Department of Natural Sciences
Longwood College
Farmville, VA 23909

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Popular version of paper 2aPAa1
Presented Tuesday morning, October 13, 1998
136th ASA Meeting, Norfolk, VA

Acoustic Shadows in the Civil War

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.
By examining war records, diaries and regimental histories, the causes of the various Civil War acoustic shadows can be examined. The battles noted for these abnormalities include Gettysburg, Seven Pines, Iuka, Fort Donelson, Five Forks, Perryville and Chancellorsville. The effects of these unusual acoustical events on the course of the war cannot be underestimated. Near dusk on May 31, 1862, General Johnston was severely wounded while reconnoitering near the front lines at Seven Pines. Johnstons mishap happened on land that would have been firmly in Confederate territory if Whiting's forces had been ordered to make their advance on time. By early the next afternoon, Confederate president Jefferson Davis had given control of the Army of Northern Virginia to the man who would lead it for the next three years - General Robert E. Lee.

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