Page 10 - Spring 2006

were easy to find. How, then, did the belief in the sound- deadening power of fog become so ingrained? It was sim- ply too easy to believe that fog would dampen sound. The temptation to compare sound to light was irresistible: light seemed to travel more or less straight and heavy rain, snow, and fog blocked its path. Sometimes sound did seem reduced in foggy weather, sometimes it didn’t, but it was easier to dismiss the contrary evidence than it was to explain it.
The belief that acoustical transparency was somehow related to optical transparency prevailed for a century and a half. But, in many ways, sound and light were already known to be quite different. Sound traveled better with the wind than against the wind, houses and hills didn’t always block sound, and distant sounds seemed clearer and louder at night than during the day.
Curious aberrations in the behavior of sound stimulated further investigation. Sometimes an event was clearly visible but the sounds were not heard. In Tyndall’s 1874 paper, he included a letter from R. G. H. Kean who had watched the Battle of Gaines’s Mill during the American Civil War2. Kean wrote, “I distinctly saw the musket-fire of both lines...I saw batteries of artillery on both sides come into action and fire rapidly. Yet looking for near two hours, from about 5 to 7 P.M. on a midsummer afternoon, at a battle in which at least 50,000 men were actually engaged, and doubtless at least 100 pieces of field-artillery...not a single sound of the battle was audible to General Randolph and myself...[However, the] cannonade of that very battle was distinctly heard at Amhurst Court-house, 100 miles west of Richmond, as I have been most credibly informed3.”
Inaudibility of loud sounds at short distances with good visibility was fatal to arguments based on transparency. Inaudibility could be explained by extreme absorption from some other mechanism. However, the “reappearance” of those sounds at extremely long distances could not be explained by absorption of any kind.
What is, at first, inexplicable stimulates explanation, debate, and experiment. As is common in the history of sci- ence, the search for understanding of the nature of sound transmission in the atmosphere started with remarkable observations. Plausible (but often incorrect) explanations fol- lowed. More observations raised more questions and stimu- lated a series of provocative and sometimes daring experi- ments. These observations and experiments eventually led to a deeper understanding not only of sound propagation but of the nature of the atmosphere itself.
Flocculence
In an attempt to bolster support for acoustic fog signals, Tyndall observed the sounds at sea from on-shore horns, sirens, and cannon fire. On one voyage he recorded that “...the rain at length reached us; but although it was falling heavily all the way between us and the Foreland the sound, instead of being deadened, rose perceptibly in power. Hail was now added to the rain, and the shower reached a tropical violence. ...In the midst of this furious squall both the horns and the siren were distinctly heard; and as the shower light- ened, thus lessening the local pattering, the sounds so rose in
pose fog.” He was confident that fog presented a substantial obstacle to the transmission of sound.
But even Derham admitted that the observations were ambiguous: “A like uncertainty obtains with regard to...foggy air. In rainy and damp weather I have often observed that sounds are blunted...but the contrary also often happens.” Unaware that a reduced level of sound could be evidence of the sound bending away from the listener instead of being absorbed, weak sounds were always explained by some sort of loss.
Widespread acceptance of the view that fog absorbed sound was damning to the case for acoustic fog signals. However, there was no other promising technology. Tyndall in England and Joseph Henry in the United States were con- vinced that a warning system for ships based on sound could be made practical and they both carried out extensive programs of experiment and observation to prove it. Tyndall attributed the prevailing views regarding the absorptive power of fog to uncritical acceptance of Derham’s work.
Before presenting his own observations as rebuttal, Tyndall collected some of the prevailing views: “Fog is a mixture of air and globules of water, and at each of the innu- merable surfaces where these two touch, a portion of the vibration is reflected and lost.” “...we must have some measure of fog’s power of stopping sound...It seems proba- ble that this will bear some simple relation to its opacity to light...” “Fogs have a remarkable power of deadening sound...” “...fogs and falling rain, but more especially snow, tend powerfully to obstruct the propagation of sound...” “That sound does not readily penetrate fog is a matter of common observation.”
As Tyndall, Henry, and others discovered, instances of excellent sound transmission through fog, rain, or snow
Fig. 1. In 1708, William Derham introduced the concept that suspended particles— whether tiny droplets of water in fog, rain drops, or snow flakes—would absorb and scatter sound perhaps in a manner similar to their effect on light. He might have drawn a figure like this where the sound waves (red) advancing to the right are either scattered from the particles or the friction between the waves and the parti- cles robs the wave of energy. Waves do scatter strongly from particles if the wave- length of the wave is similar in size or smaller than the particles and this is the case for light. However, the wavelengths of audible sound waves are far larger than the particles of fog, rain, or snow so the scattering is much too weak to explain the observations of poor sound reception.
8 Acoustics Today, April 2006