Refraction of sound in the atmosphere

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 recognize 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 ignorance, speculation, and misunderstanding 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...