936 MISCELLANEOUS GEOPHYSICAL METHODS [Chap. 12 



and explosion observations and is similar in principle to the "reflection" 

 of radio waves from the Kennelly-Heaviside layer. Another cause of ray 

 curvature may be the increase of wind velocity with altitude. 



Sound is absorbed in air because of viscosity, heat conduction, and 

 scattering on small particles. The effect of viscosity and heat conduction 

 may be expressed by 



where I is intensity at distance x and a is an amplitude attenuation coeffi- 

 cient, that is, the reciprocal of the distance at which the amplitude has 

 dropped to 1/e of its initial value. If internal friction alone is considered, 

 the coefficient 



« = |^', (12-17) 



where n/5 is the static mass-viscosity coefficient, v is the sound velocity, 

 and / is the frequency. The coefficient of absorption due to heat con- 

 duction is about one- third of that due to internal friction. The absorption 

 increases in proportion to the square of the frequency; hence, the range of 

 audio signals in air is much less (30 kilometers maximum for the band of 

 300 to 600 cycles) for audio frequencies than that of explosion sounds 

 (which have been recorded by microphones up to 400 to 500 km at 5 to 10 

 cycles). However, there are so many possible interferences with sound 

 transmission, due to variations in meteorologic factors (humidity, wind, 

 clouds, fog, and the like) that these ranges are not always reached; sound 

 transmission through air is, therefore, much less reliable than through 

 water. Some of the very large ranges observed for explosions are no doubt 

 caused by the so-called "abnormal" sound propagation, that is, repeated 

 reflections on the (ozone) layer and the earth's surface. For very high 

 (supersonic) frequencies the absorption and scattering on small particles 

 become so strong that their range is very small. The scattering effect is 

 inversely proportional to the volume of the particles concerned and directly 

 proportional to the fourth power of frequency. 



2. Sound transmitters. The design and construction of sound trans- 

 mitters and receivers for atmospheric acoustic work depends entirely on 

 the purpose for which the sound transmission is intended. When sound 

 originates without control by the listener (explosions, gun fire, airplane 

 propeller noise) the technical problem is, of course, confined to the con- 

 struction of suitable receivers. In atmospheric acoustic signaling, position- 

 finding, and echo-sounding, the transmitters vary in construction, but the 

 emphasis is usually on the low-frequency end of the spectrum. The Behm 

 airplane echo sounder and the "Echometer" for measuring the depth of 

 fluid levels in wells employ simple gunpowder cartridges as transmitters. 



