2. Attenuation Loss . This is a second form of loss of intensity of 

 a sound wave as it propagates in a given medium such as water. The total 

 loss, out to a given distance, r, is the sum of the spreading loss and 

 attenuation loss, each expressed in decibels. Attenuation loss results from 

 sound power being extracted from the wave as it propagates. The extraction of 

 power results when some of the energy of the wave is scattered in various 

 directions by various inhomogeneities in the medium, such as small organisms, 

 gas bubbles, etc. Loss of power also occurs when some energy is absorbed by 

 molecular interactions, and is converted into heat. In either case the 

 attenuation loss is proportional to the intensity of the wave, and the loss is 

 given by a certain fraction of the intensity per unit distance of travel. 

 This fraction is known as the attenuation coefficient per meter or per foot, 

 or per kiloyard. it is usually expressed in decibels as so many decibels per 

 unit distance. Figure 2 shows values of the attenuation coefficient for sea 

 water in decibels per kiloyard. These particular units are much used in 

 underwater sound. 



These two components of sound propagation loss discussed above are 

 treated quantitatively as follows: 



1. For the most common form of spreading, spherical spreading, the 

 wave front is an ever-expanding sphere, and the spreading loss is 6 decibels 

 (dB) per doubling of distance (inverse square law). This was described in the 

 example above. A second common form, where the sound is confined between two 

 reflecting planar boundaries (the surface and a reflective bottom) is called 

 cylindrical spreading. Here the spreading loss is proportional to the first 

 power of distance, so it is 3 dB per doubling of distance. 



G-15 



