FUTURE RESEARCH 



243 



anism. Among the dissipative mechanisms consid- 

 ered are compression viscosity (which, however, cer- 

 tainly is not the principal factor at 24 kc), gas bub- 

 bles present in the water, fish bladders, plankton, and 

 thermodynamically irreversible chemical reactions, 

 such as the hydrolysis of dissolved salts. 



Gas bubbles and other inhomogeneities would not 

 only absorb but also scatter sound. That scatterers 

 are present in the sea is known. Scattering may ac- 

 count for part of the attenuation of highly collimated 

 beams and also is probably responsible for most of 

 the sound observed in predicted shadow zones in the 

 presence of negative velocity gradients. 



All hypotheses concerned with the cause of the 

 absorption of sound as well as with the role of volume 

 scattering on sound transmission are at present 

 largely speculative. Until further experimental and 

 theoretical work has provided a scientific under- 

 standing of the mechanisms involved, it will not be 

 possible to predict with confidence the attenuation 

 under many different conditions. 



Surface Reflection 



The change in density at the sea surface is known 

 and is so large that for most practical purposes the 

 density of air may be set equal to zero; that is, the 

 surface is almost a perfect reflector of sound. The 

 complexity of surface-reflected sound arises from the 

 complicated form of the ocean surface. In principle, 

 it is simply a mathematical problem to compute the 

 sound reflected from any surface of known properties. 

 In practice, observations are unquestionably re- 

 quired. A thorough understanding of this topic would 

 be important in studies both of fluctuation and of the 

 average transmission anomaly in the surface layer. 



Bottom Reflection 



The ocean bottom may have a topography equally 

 as complicated as the ocean surface. In addition, the 

 relative change in the elastic parameters and in 

 density across the interface is much less extreme 

 than across the ocean surface, and the detailed values 

 of these changes must be considered. Since the physi- 

 cal properties of the bottom may vary with position, 

 both vertically and horizontally, the problem of bot- 

 tom-reflected sound can be very complicated physi- 

 cally as well as mathematically. In certain regions, 

 where the bottom is flat, and of uniform composition, 

 the acoustic phenomena are perhaps capable of being 

 understood. Bottom-reflected sound is obviously im- 



portant in many situations, especially when the direct 

 sound is weakened by temperature gradients. 



10. .5. 2 



Methods 



To understand the physics of underwater sound 

 transmission, each problem must be given separate 

 consideration. The following methods may be ap- 

 plicable, however, to the investigation of a con- 

 siderable variety of problems. 



OCEANOGRAPHIC MEASUREMENTS 



An important part of any basic research on sound 

 in the sea must be the investigation of the physical 

 properties of the medium in which the sound is trans- 

 mitted. It is in terms of these properties that the 

 acoustic data are presumably to be interpreted. 



In the first place, detailed measurements of the 

 factors influencing sound velocity seem desirable, 

 especially temperature measurements showing the 

 full detail actually present in the sea. In the second 

 place, detailed measurements of the shape of the 

 ocean surface are required before any attempt can 

 be made to explain surface-reflected sound; in par- 

 ticular, statistical information on the spectrum of the 

 surface water waves present during any interval 

 seems desirable. In the third place, complete physical 

 data on the ocean bottom (on topography, composi- 

 tion, porosity and compactness, etc.) are required to 

 interpret physically the data on bottom-reflected 

 sound. Finally, it may be necessary to make a variety 

 of physical measurements on ocean water as part of 

 the attempt to identify the cause of absorption. 



Controlled Acoustic Measurements 



The experimental techniques of underwater acous- 

 tics research will probably be developed in a number 

 of directions. Greater emphasis may be expected 

 on detailed accuracy of the acoustic data; probable 

 errors of several decibels for a transmission anomaly 

 can presumably be considerably reduced. Measure- 

 ments involving smaller samples of the ocean may 

 perhaps be anticipated with relatively complete 

 oceanographic data obtained for the small samples 

 investigated. Some such experiment might be devised 

 for measuring the sound absorption in a relatively 

 small volume. Another possible development is along 

 the lines of multiple measurement, in which many 

 different items are measured almost simultaneously. 

 For example, the inclination of the wave front might 

 be measured at the same time as its intensity with 



