312 EARLEC. GREGG, JR. 



sound wave would have to travel 46 cm. before its intensity would be 

 reduced to one-half its initial value. 



It is important to note that the coefficients in Table II are for 

 homogeneous isotropic materials. The presence of foreign matter will 

 increase the absorption coefficient not only by absorption of energy 

 by the foreign matter itself, but also by scattering the sound energy 

 out of the beam. This effect of foreign matter (particularly dissolved 

 gases) on the absorption of energy is of extreme importance in bio- 

 logical work. More will be mentioned about this later. 



7. Cavitation and Degassing 



Whenever a sound wave traverses a liquid in which there are dis- 

 solved gases, small groups of gas bubbles are formed. For sound of 

 low energy, it has been found that these bubbles are caused by the 

 union of microscopically small bubbles as they move toward the 

 nodes of a stationary wave. For larger energies, the negative pres- 

 sures involved actually cause the emergence of the gas dissolved in 

 the liquid and greatly increase the rate of bubble formation. Even 

 with no dissolved gases, the large negative pressures and hence large 

 stresses in the liquid w^ll cause small hollows or cavities to be formed, 

 which then become filled with the vapor of the surrounding liquid. 

 These hollows of course disappear when the sound beam is turned off. 

 The formation of these hollows by the literal tearing apart of the 

 liquid is known as cavitation. 



If there are dissolved gases, the local action caused by the gas 

 filling these cavities is tremendous and explains in part most of the 

 observed biological and chemical actions of ultrasound. There is not 

 only tremendous local agitation but also high local temperatures and 

 possibly electrical potentials due to the frictional losses involved as 

 the gases escape into the cavities. The absorption coefficient under 

 these circumstances is very large. Investigation has shown that the 

 power per square centimeter required to produce cavitation (and 

 hence bubble formation) depends mostly on the external pressure 

 while the energy required to produce a given volume of gas depends 

 on both the external pressure and frequency. 



In general, cavitation occurs in a light liquid filled with air when 

 the sound pressure is on the order of the hydrostatic pressure at the 

 point in question plus the external pressure on the liquid. For ex- 

 ample, for water at atmospheric pressure and negligible depth, the 

 acoustic intensity required to produce cavitation is : 



