Page 9 - Spring 2006

REFRACTION OF SOUND IN THE ATMOSPHERE
Thomas B. Gabrielson
Applied Research Laboratory, The Pennsylvania State University State College, Pennsylvania 16804
surprising that in dense fogs our most powerful coast-lights...should become useless to the mariner...Disastrous shipwrecks are the consequence. During the last ten years no less than two hundred and seventy-three vessels have been reported as totally lost on our own coasts in fog or thick weather. The loss, I believe, has been far greater on the American seaboard, where trade is more eager and fogs more frequent than they are here. No wonder, then, that earnest efforts should have been made to find a substitute for light in sound-signals, powerful enough to give warning and guidance to mariners while still at a safe distance from the shore.”
Understanding the behavior of sound was not merely an intellectual pursuit; lives and commerce were at stake: sound was a logical supplement for lighthouses when visibility was poor. However, the prevailing belief was that sound traveled poorly through fog (and rain and snow). If sound was absorbed by fog, what good would it be as a guide to mariners? In fact, sound does penetrate fog, often quite well, but the contrary belief had become the “expert opinion” from the early 1700’s well into the 19th century. How did this happen?
Transparency
In 1708, Reverend William Derham published, in Latin, what became the definitive summary regarding the transmission of sound. Many of Derham’s observations were accurate and his comments insightful; however, parts of the paper are conjec- ture with little supporting observa- tion. Regarding fog, Derham wrote, “...as regards thick fogs, it is certain that they are dampers of sound in the highest degree...a fact that very cer- tainly proceeds from the interposed vapors and thick particles that com-
“Understanding the behavior of sound was not merely an intellectual pursuit—lives and commerce were at stake...”
Introduction
It is natural to think that the path sound travels is more or less straight. Certainly sound “leaks” around buildings and reflects from walls but, for the most part, when you hear a sound, you know where to look to find the source.
You may not realize you’ve made this assumption but it still affects your interpretations. If you watch a cannon being fired and the sound seems unusually weak, then surely it “must” be the result of some extra loss of energy from the sound wave as it traveled. You might develop an entire theory of sound absorption based on such observations.
But, what if the path of sound isn’t straight? What if the sound from the cannon actually curved upward and passed over your head? Then the sound would only appear to be absorbed when in fact it had simply changed direction. No matter how plausible your theory of absorption might be, it would be wrong.
As the science of sound developed in the 18th and 19th centuries, this is just what happened. As unlikely as the idea of bending of sound paths seems, we now know that this bending— refraction—is commonplace. The speed at which a sound wave travels in air depends primarily on temperature and on the speed and direction of the wind. If the speed changes from one point to another along the “crest” or “front” of a sound wave, then the wave-front bends as it moves forward. For sound in the atmosphere, the effects of refraction are dramatic and nearly always present. But, in spite of a flurry of activity, the failure to recog- nize refraction slowed progress in understanding sound to a crawl.
The latter half of the 19th century was a watershed for understanding the science of sound in the atmosphere.
Short-sighted focus on absorption and reflection gave way to understanding and acceptance of refraction as the dominant mechanism. But the context for the story of refraction spans about 250 years from the early 1700’s to the middle of the 20th century. The story is a reminder that science is a human pursuit. The story started with igno- rance, speculation, and misunder- standing but it led eventually to a deeper understanding of sound and of the structure of the atmosphere itself. And, the story started on a dark and stormy night...
Fog
Returning from France in fog late one October evening in 1707, four ships of the British fleet struck the Outer Gilstone Ledges and sank southwest of the Scilly Isles. The loss of the ships and more than one thou- sand sailors eventually prompted the British Parliament to pass the Longitude Act of 1714 that called for drastic improvement in navigation at sea. The history of the race to deter- mine location at sea is captivating1; however, this navigational “solution” did not prevent shipwrecks in coastal fog. The required observations of, for example, the sun required good weather. Sailing between these often infrequent “fixes” still relied on the educated guesswork known as dead reckoning. In the fog, ships still sank.
Even though shipboard naviga- tion was much improved by 1874, John Tyndall still wrote “...it is not
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