sound velocity, and it has also demonstrated that the atten- 

 uation in sea water is extremely small, when the water is 

 free of contamination. By way of illustration, Horton* 

 states that the attenuation coefficient in pure sea water is 

 approximately 0. 5 db/kyd at 5000 c/s. 



Studies carried out by the direct method 5-7 clearly 

 demonstrate the difficulties involved. Chief among these 

 difficulties is that of assessing the influence of plankton. 

 Plankton is far from homogeneously dispersed in the ocean. 

 It occurs frequently in quite sharply defined layers, and is 

 often very patchy within the layer. To determine the 

 acoustic attenuation by plankton from an interpretation of 

 measurements obtained by the direct method is therefore 

 virtually impossible and has not been attempted. 



A few methods classified here as laboratory procedures 

 have been modified for field use. A method of velocity 

 measurement by spherically propagating sound waves has 

 been adapted for field measurements. 8 However, in this 

 system, the path length involved is too short for obtaining 

 any useful information about attenuation. Adaptation of 

 the reverberation tank to field use is more pertinent to the 

 attenuation problem. 9 



The direct field methods have served mainly in identi- 

 fying certain propagation anomalies in a gross sense and 

 in emphasizing the need for more detailed studies. Such 

 anomalies as the deep scattering layer, effect of ships' 

 wakes, "quenched water, " "knuckles, " and "black-outs"** 

 have been detected by direct methods. 



LABORATORY METHODS: PLANE WAVE PROPAGATION 



Laboratory methods require only a relatively small 

 liquid sample and therefore lend themselves to measure- 

 ments of the acoustic properties of more or less exotic 



*Ref. 1, p. 81 

 **Ref. 1, p. 82 



