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long-range effects. Beno Gutenberg wrote: “In 1904, G. von dem Borne investigated a region where an explosion of dyna- mite had been heard. He found that there were two distinct zones of audibility, one surrounding the source and another, separated from the first by a “zone of silence,” at a greater dis- tance. Many subsequent investigations have shown that usual- ly two or more zones of audibility result from explosions.” “The data...have led to the conclusion that the sound waves that arrive at the second and succeeding zones have traveled through the stratosphere and that their velocity at the highest point exceeds the velocity of sound near the ground.” “The outer boundary of the [second] zone is fixed by the rays that have their highest points between 60 and 70 km above the ground. As Schrödinger has shown no sound can be transmit- ted at greater heights because the distance between the mole- cules is too large there.”
(Erwin Schrödinger6, published an account in 1917 of what we now call classical absorption—the absorption of sound from viscosity and thermal conductivity. Schrödinger calculated high levels of absorption at extreme altitudes; so high that refraction of audible sound from heights above 60 km or so would not be observed.)
In order to refract sound back down to earth more than 100 km from the source, the refraction must have been tak- ing place at higher altitudes than could be explained by refraction from surface winds or near-surface temperature gradients. Not only that, the zones of audibility seemed to extend further to the east in the winter and further to the west in the summer. The success of refraction in explaining propagation of sound over shorter distances suggested look- ing for refraction at higher altitudes. But what was the cause: wind, temperature, or something else?
The road to discovery continued to twist, turn, and even reverse direction. Up to this time, balloon observations of
Fig. 10. Observations of unusually long-distance propagation of sound, especially dur- ing World War I, led to a series of ambitious experiments after the war. Large explo- sions were set off and observers reported whether or not they heard the explosion. The results of one such explosion at Kummersdorf (south of Berlin) on December 18, 1925 are shown here. The cross-hatched regions in the figure above are those regions in which the explosion was heard. The ring-like structure with zones of audibility alternating with zones of silence suggested that the sound was reflecting from the ground and then being refracted back down to the ground over and over. The distance between rings suggested that the refraction was taking place in the stratosphere. (Adapted from B. Gutenberg, Handbuch der Geophysik with the base map used by permission from The Historical Atlas by William R. Shephard, 1911.)
Fig. 9. Near the ground, the air temperature gradient (its change with height) often changes from night to day. The upper graph shows the variation in temperature sam- pled every ten seconds for three sensors located at 0.2 (blue), 2 (black), and 4 (red) meters above the ground. At night, the ground cools by radiation into the clear sky so the temperature of the air above the ground increases with height. After sunrise, solar heating of the ground reverses the temperature gradient: the temperature at 0.2 meters (blue curve) is higher than that at 4 meters (red curve). The spike after sun- rise is probably a small convection current—it was accompanied by a noticeable breeze. The temperature gradient every ten minutes is shown in the lower plot. A line leaning to the right (and colored red) indicates an increase in temperature with altitude (an “inversion”) whereas a line leaning to the left (and colored blue) indicates a decrease in temperature with height. (Data taken by the author.)
example—begged for similar resolution. Furthermore, the roles started to reverse. At first, the vagaries of sound propa- gation prompted investigation of the atmosphere. Gradually, however, sound became a valuable tool for understanding the properties of the atmosphere.
Into the stratosphere
With the recognition of refraction by either wind or temperature gradients, Kean’s inability to hear the battle at Gaines’s Mill now had a reasonable explanation: little or no wind was reported5 but the normal temperature decline with altitude produced sufficient upward refraction to pre- vent Kean from hearing the sounds of rifle and cannon fire. Why, though, were the same sounds heard 100 miles away? If refraction could explain the short-range observations, could it also account for the more distant ones? But what would bring the sound back down at such long ranges?
As common as refraction of sound might be, it took unexpected observations to make people notice. World War I provided a tragically constant source of powerful explosive sounds. Gunfire in France might not be audible within a few kilometers of the source but might be heard hundreds of kilo- meters away in England.
Isolated large explosions provided additional evidence for
14 Acoustics Today, April 2006