The above factors, as they stand, represent attenuation 

 in units of neper per unit time. In measurements the rever- 

 beration time is determined. This represents the time that 

 it takes the sound level to die down 60 db, measured in 

 seconds. Thus if t denotes the reverberation time as 

 measured with the cavity filled with pure water and ^+ + a i 

 the reverberation time as measured with a sea water 

 sample, then 



(a*) 



t'e: 



60 



total 



(79) 



the units being db/sec. 



In practice it is the attenuation per unit length along 

 the propagation path which is desired. From the considera- 

 tions of the plane sound waves it is evident that the conver- 

 sion to the spatial attenuation factor, a^, for the fluid is 

 simply a^ = a^/c. For sea water of 34 gms/kg salinity at 

 60°F c = 1.645 kyd/sec; hence the excess attenuation 

 expressed as db per kyd is given by 



36.5 



'total 



(80) 



This relation has been used in the evaluation of the data. 



It is theoretically possible to determine the sound 

 velocity from the resonance frequency, but the geometrical 

 dimensions of the cavity are not very stable, since it is 

 made of thin sheet metal with welded corners. The accu- 

 racy of the frequency measurements were only as good as 

 can be obtained by reading the scale on the tone generator, 

 and the determination of o on this basis is not precise. 

 The temperature range encountered was from about 10°C 

 to 2 0°C, and the sound velocity changes about 2 per cent 

 due to this temperature variation, but it is not known how 

 the presence of plankton may change c. It was therefore 

 decided to use a constant value for a, to be able to give a^ 



58 



