II-5 



Not only does the air bubble remove energy from the incident wave to 

 produce the isotropic scattered wave, but it also dissipates a certain amount of 

 energy due to viscosity, heat conduction, and surface tension. All this energy 

 is removed from the incident wave, and we can make theoretical estimates of 

 the resulting extinction cross section for a single bubble. 



Numerous experiments have been conducted to compare the resonant 

 and dissipative behavior of single bubbles with the theoretical predictions. The 

 theoretical frequencies have been found to check exceedingly well with experi- 

 mental results. The dissipative properties of single air bubbles check less well 

 with the theory, and there is substantial difference between the results obtained 

 in different experiments. Nonetheless, on the average, the theory seems to 

 predict the experimental results. 



In most portions of the ocean, air bubbles (including the gas swim 

 bladders of fish, which behave like air bubbles) are sufficiently widely spaced 

 that the single scattering approximation is adequate. However, regions with a 

 higher concentration of air bubbles do occur (for example, in fresh wakes) and 

 require a treatment incorporating the effect of multiple scattering. These bubble - 

 water mixtures exhibit a cooperative behavior which makes them act macro- 

 scopically as a homogeneous medium with a sound velocity different from that of 

 pure water and with substantial attenuation of the sound intensity. The theoiy of 

 these bubbly mixtures is presented, and compared with the available experimental 

 data. A substantial number of experiments have been performed, but all under 

 controlled laboratory conditions rather than in the ocean. In view of the experi- 

 mental difficulties encountered in producing bubble screens of uniform properties, 

 the agreement between theory and experiment is quite good. 



Section III-C treats scattering from marine organisms; these are 

 idealized either as a sphere of a different fluid or else as a sphere of different 

 fluid contained in an elastic shell (e.g., crustaceans). Obviously, the spherical 

 model does some violence to reality, but one might hope that the model would 

 suffice to explain the main features of scattering from a collection of organisms 

 all of somewhat different shapes. Unfortunately, the experimental evidence 

 from marine organisms is inadequate to draw conclusions about the theoretical 

 calculations. 



One interesting aspect of the ocean which might be explainable in terms 

 of the behavior of marine organisms is the so-called deep scattering layer. It 

 appears that the properties of this layer, which change with time of day, can be 

 attributed to scattering from fish with gas bladders. The plausibility of different 

 hypotheses regarding the contraction and expansion of these swim bladders as the 

 fish change depth are discussed in the light of the resonant frequencies that would 

 result. 



artbur a.littlcJnt. 



S-7001-0307 



