II. METHODS OF MEASUREMENT 



A great amount of work on the determination of sound 

 absorption in liquids exists in the literature and it is 

 clearly necessary to limit the discussion of methods of 

 measurement to those aspects which are most pertinent 

 to the present investigation- -specifically, the influence 

 of biological entities and air bubbles on sound propagation 

 in sea water and the methods of measuring them. The 

 references cited here have been chosen with the aim of 

 presenting a representative cross section of the recent 

 work in this field. There exist several articles and texts 

 covering the broad aspects of the problem of sound attenu- 

 ation. 1_4 (See list of references at end of report. ) 



Attenuation can be measured by (1) field methods, 

 taking place in the ocean where the disturbance of the 

 sample can be kept at a minimum; and (2 ) laboratory 

 methods, which involve taking samples out of the ocean 

 to perform the measurements. 



FIELD METHODS 



The direct(and the simplest) method consists of intro- 

 ducing two hydrophones in the ocean and performing trans- 

 mission measurements. The inherent difficulty is that the 

 measurements are influenced by a large number of vari- 

 ables, and it is difficult to unscramble the combined effects 

 so as to achieve dependable conclusions about the influence 

 of the plankton population. 



For clear sea water the attenuation is small and it is 

 therefore necessary to use long distances between trans- 

 mitting and receiving hydrophones. Moreover, the influence 

 of thermal stratification and of reflections and losses at 

 surface and bottom, as well as the intensity changes due 

 to spreading of the acoustical beam, must be accurately 

 known. The direct method has been used to determine the 



