I. INTRODUCTION 



It is well known that the propagational characteristics 

 of acoustical energy depend upon the type of material 

 medium and the distribution of inhomogeneities within the 

 medium. An abrupt change in the ocean medium as occurs 

 at the sea surface or sea bed produces reflection or rever- 

 beration of acoustical energy. Variations of sound speed 

 in the sea produce refraction effects. The presence of 

 air bubbles or suspended material, including plankton, 

 will produce scattering of the sound energy. Viscosity, 

 diffusion, certain thermochemical effects, and other non- 

 reversible processes can lead to absorption of sound energy. 



The biota of the sea can also influence the acoustic 

 properties to a marked degree. Schools of fish not only can 

 give rise to echoes, but can also impair the transmission 

 of a sonic beam of energy from point to point by scattering 

 the energy out of the beam. The best known effect of this 

 sort is perhaps the deep scattering layer. It appears on a 

 fathometer record as a false bottom above the true bottom 

 and in view of its vertical migration during the day, is 

 thought to be caused by a layer of organisms. 



In recent years there has been an interest in the prop- 

 erties of low-frequency acoustic energy in the sea. Theo- 

 retically, the viscous attenuation coefficient of acoustic 

 energy in a plane sound wave is directly proportional to the 

 square of the frequency. However, in view of other sources 

 of attenuation, particularly the scattering and absorption 

 associated with air bubbles and plankton, it is expected 

 that anomalous attenuation can occur. The present report 

 is concerned with the problem of isolating some of the 

 factors producing attenuation of low-frequency acoustic 

 energy in sea water. 



Several of the above mentioned inhomogeneities of the 

 medium are almost always present at the same time. This 

 makes it difficult in field measurements to separate the 

 different effects of temperature and salinity gradients, 



