2. The second form of sound transmission loss, attenuation, 

 results, as mentioned above, from loss of energy by such processes as 

 absorption by molecular interaction in sea water, and scattering of energy by 

 inhomogeneities such as marine organisms and bubbles. Attenuation loss is 

 very dependent on frequency, as is shown in Figure 2, and slightly dependent 

 on temperature. 



Figure 2 shows the attenuation coefficient in dB per kiloyard 

 plotted as a function of frequency. Note the very low loss at low frequency 

 (.001 dB/kiloyard at 0.1 kilohertz). This leads to very long range 

 transmission of low frequency sound. For example, sound at 100 hertz is 

 attenuated only 1 dB over a distance of 1000 kiloyards (493 nautical miles). 

 Of course, one must recognize that the spherical spreading loss from a range 

 of 1 yard to 1000 kiloyards is 120 dB, so at low frequencies, where 

 attenuation loss is small, spreading loss is dominant. 



The equations for calculating transmission loss (TL) as a 

 combination of spreading loss and attenuation are, for TL in decibels: 



For spherical spreading: TL = 20 log r + ar x 10-3 (1) 

 For cylindrical spreading: TL = 10 log r + ar x 10-3 (2) 



where 



r = range in yards 



a = absorption loss in dB per kiloyard 



G-16 



