Page 18 - Spring 2006

Density measurements offer a less intuitive but more reliable method for high-altitude temperature determina- tion. The temperature can be calculated from the density if the altitude and molecular weight of the air are known. The density can be inferred from a dynamic pressure (“pitot”) probe on a rocket. A more creative approach is to eject an inflatable sphere during the rocket’s ascent. Aerodynamic drag on the sphere controls its subsequent path. By tracking the trajectory of the sphere, the density can calculated from the sphere’s acceleration and drag coefficient. (Osborne Reynolds would be pleased: the coefficient of drag for a sphere is a function of the “Reynolds Number.”)
The study of atmospheric sound from 1700 to the first half of the 20th century is a fascinating story of the real sci- entific method—the human method—with the foibles and mis-steps, arguments and experiments, errors and correc- tions of science and scientists. Derham’s “expert opinion” in the early 1700’s hindered understanding for more than a century. Scientists in the mid-1800’s rebutted these opin- ions but replaced them with other errors: the new explana- tions were attractive and plausible but wrong. Later, evi- dence for refraction overwhelmed other arguments and the often strange observations were reconciled. The hydrogen atmosphere came and went but, ultimately, the study of sound exposed the general structure of winds and tempera- ture in the upper atmosphere well before direct measure- ment was possible.AT
Comments
1. Dava Sobel’s Longitude (1995) is an excellent popular account. Rupert Gould’s The Marine Chronometer (1923) is harder to find but worth reading for the technical detail.
2. Many other fascinating accounts of the influence of acoustic propagation on battle strategy in the Civil War are contained in C. D. Ross, Civil War Acoustic Shadows (White Mane Books, Shippensburg, PA, 2001) and, by the same author, “Outdoor sound propagation in the US Civil War,” Applied Acoustics 59, 137-147 (2000).
3. Tyndall did not embrace the concept of refraction. It is curious that Kean’s letter is reproduced apparently in its entirety in Tyndall’s 1874 paper but the remarkable statement about hear- ing the battle sounds 100 miles away was omitted when Tyndall quotes from the letter in his 1908 book, Sound.
4. Atleasthewasquestioningtheprevailingviewsandchallenging the scientific literature.
5. Artwork from the battle and reports from the Signal Corps agree that the smoke of the battle dispersed very slowly; however, it is not out of the question that a light wind played some role in the refraction.
6. The story of the development of atmospheric acoustics is inter- esting not only for the science but also for the people. Reynolds is best known not for his work in acoustics but for his work on turbulence in fluid flow and Schrödinger for his work on quan- tum mechanics. Werner Heisenberg—also a name from quan- tum mechanics—published on atmospheric science: he found a lower limit to the size of eddies in turbulence in the atmosphere.
7. If buoyancy alone were the controlling factor, argon—heavier than either oxygen or nitrogen – would produce an uninhabit- able blanket of gas near the ground. But Lord Rayleigh didn’t publish his discovery of argon until 1894.
8. Thisstatementgivestheimpressionthatozoneisanaturalcom- ponent of the atmosphere that happens also to absorb solar radi-
producing these high temperatures.”
While other methods of atmospheric research were being
developed, refraction of sound remained one of the corner- stones. Any proposed structure for the atmosphere that did not explain long-range refraction of sound was unacceptable.
Into the 20th century
By the early 1900’s, refraction of sound in the atmos- phere was accepted widely. Extensive direct measurement by balloons, kites, and airplanes had exposed the major temperature and wind variations with altitude in the lower atmosphere. Innovative indirect observations of sound, meteor trails, and high-altitude clouds had established the gross features of temperature and wind variations in the stratosphere. By the end of World War II, high-altitude bal- loon flights confirmed the already suspected seasonal wind direction reversal in the stratosphere—flow toward the west in the summer and toward the east in the winter.
Infrequent and expensive manned balloon flights gave way to regular measurements from unmanned balloons, airplanes and, later, rockets. Sounding rockets probed alti- tudes much higher than balloons but the problem of tem- perature measurement was not solved merely by flying a temperature sensor on a rocket. At extreme altitudes the sensor may reach thermal equilibrium with solar radiation instead of with the tenuous air surrounding the sensor. Furthermore, any temperature reading must be corrected for the compressional heating produced by the fast-moving nose of the rocket.
Fig. 12. The seasonal change in wind direction and speed in the stratosphere was also determined from measurement of sound propagation from explosives. A. P. Crary determined these wind speed profiles for the summer (red) and winter (blue) in Alaska in 1948 and 1949. The points indicated by the circles and x’s were deter- mined from acoustic measurements. The regions below 20 km are representative of historical measurement by balloon. Many other experiments verified this general behavior—the reversal in direction of stratospheric winds from summer to winter. (The Jet Stream occurs at altitudes generally between 10 and 15 km in the region of non-reversing winds.) The temperature and wind speed variations in the upper atmosphere explained both the occurrence of zones of silence and zones of audibil- ity and the shifting of those zones with season. (Adapted from Crary, 1950.)
16 Acoustics Today, April 2006