242 



SUMMARY 



about as far as usefulness requires, and future studies 

 will most profitably be directed to a more funda- 

 mental investigation of the basic factors underlying 

 the observed data of underwater sound transmission. 

 Without such a reorientation of the basic research 

 program, it will be impossible to predict the behavior 

 of underwater sound under new and unexplored con- 

 ditions. Suppose, for instance, that sound gear using 

 a nondirectional supersonic projector were to be pro- 

 posed. The transmission loss for the sound from such 

 a system could not be predicted definitely from pres- 

 ent data, which are all obtained with directional 

 supersonic sources. To make such predictions would 

 require some knowledge of the importance of the 

 scattering of sound through small angles. Similarly, 

 the attenuation of sound transmitted from a deep 

 projector to a deep hydrophone cannot be predicted 

 from the present empirical data taken with shallow 

 projectors, but might be estimated if the basic causes 

 of attenuation were known. 



In principle, the answer to any practical question 

 about underwater sound transmission could be ob- 

 tained by a program of measurements planned wholly 

 for the purpose of answering that question. When 

 haste is required, this is frequently the quicker 

 method. When time is available, however, such 

 answers can most efficiently be provided by a broad 

 program designed to yield a physical understanding 

 of what is happening. Such a program makes it ulti- 

 mately possible to answer not one but a large number 

 of practical questions. Thus, in the long run, im- 

 proved technology can best be based on a foundation 

 of long-term fundamental research. 



This final section gives a brief discussion of some 

 of the basic physical factors that may be expected to 

 be important in underwater sound transmission and 

 also treats the type of observations that might be 

 expected to give meaningful information on these 

 diiferent factors. 



10.5.1 



Basic Factors 



The wave equation, equation (27) of Chapter 2, 

 presxmiably governs in good approximation the prop- 

 agation of sound waves in the interior of the ocean. 

 It appears reasonable at first to investigate solutions 

 of the simple wave equation, taking account of the 

 presence of velocity gradients in the sea and of the 

 reflections from sea surface and sea bottom. If the 

 results are in flagrant disagreement with observa- 

 tions, then the effects of the approximations entering 



into the derivation of the wave equation must be in- 

 vestigated in detail. Apart from the validity of the 

 wave equation as such, it is known that the body of 

 the ocean contains scatterers (their nature uncertain) 

 which deflect a fraction of the sound energy from its 

 original direction of propagation. Furthermore, the 

 observed absorption at supersonic frequencies far 

 exceeds the value predicted on the basis of viscosity 

 alone, necessitating the assumption of additional 

 dissipative processes. 



The most important problems of underwater sound 

 transmission may thus be summarized under the 

 following four headings. 



1. The effects of velocity gradients in the sea. 



2. Absorption and scattering in the volume of the 

 sea. 



3. Surface reflection. 



4. Bottom reflection. 



Each of these topics is discussed in the following 

 subsections. 



Sound Velocity 



The velocity of sound is known as a function of 

 temperature, pressure, and salinity and thus can be 

 calculated at any point in the ocean where these 

 physical quantities are known. The refraction effects 

 produced by smooth vertical changes of temperature 

 have been extensively investigated theoretically, and 

 the results are in general qualitative agreement with 

 the observations. Since the agreement is not com- 

 plete, however, other effects must also play an im- 

 portant part. While the pressure is known as a func- 

 tion of depth, changes in temperature and salinity 

 over distances of a few feet have not been extensively 

 measured, and the acoustic effects to be expected 

 from such changes have not been thoroughly ex- 

 plored. Microstructure of temperature and perhaps 

 also of salinity may have an important effect on sound 

 transmission, especially when the smoothed vertical 

 gradient of sound velocity is small. Also, microstruc- 

 ture probably accounts for some part at least of the 

 observed fluctuation of transmitted sound. 



Absorption and Scattering 



The attenuation observed in deep isothermal water 

 is presumably the result of absorption, that is, some 

 dissipative process which converts sound energy into 

 heat. Since the attenuation observed at 24 kc exceeds 

 by a factor of about 100 the value predicted on the 

 basis of shear viscosity alone, the principal cause of 

 the observed attenuation must be some other mech- 



