B. Sound Propagation 



As noted earlier, the calculation of sound transmission loss for 

 various distances from an underwater source can be very complex and involve 

 many oceanographic factors related to the water depth, temperature structure, 

 bottom type, etc. Basically, sound transmission loss can be considered as 

 made up of two components: (1) spreading loss, which results from the energy 

 in the wave front being spread out over a greater total area as the sound 

 travels, and (2) attenuation, which is the loss of sound energy due to 

 absorption and scattering in the medium. 



The following discussion is presented as an aid in understanding the 

 nature of spreading loss, and attenuation loss. 



1, Spreading Loss . Assume that a sound radiates one watt of sound 

 power into the ocean, and that this sound radiates equally in all directions 

 (omnidirectional source). The sound wave-front propagates outward from the 

 source as an ever-expanding sphere, with an ever-increasing surface area 

 'hich, at distance r from the source, is given by 4(rr2. The intensity of 

 the sound is defined as power per unit area of the wave. The unit of source 

 intensity is watts per square meter. For the above example of a source power 

 of 1 watt, the intensity at a distance r = 1 meter is 1 divided by 4ir = .08 

 watts per square meter. At a distance of 2 meters, the intensity is 1 divided 

 by 4tt(2)2 = .02 watts per square meter. Thus, doubling this distance 

 reduces the sound intensity by a factor of 4. Expressed in decibels, this 

 reduction is 10 log 4=6 decibels. 



w 



G-14 



