THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

15.8 Underwater Refraction 421Figure 15.5. Attenuation and absorption processes in the sea.15.7 Spherical Spreading Combined with AbsorptionIt has been found in propagation measurements made at sea that spherical spreadingtogether with absorption provides a reasonable fit to the measured data under asurprisingly wide variety of conditions, even in situations where spherical spreadingis not supposed to occur (e.g. trapping conditions in sound channels). Whenan approximation of the transmission loss TL is sufficient, the universal sphericalspreading law plus the loss due to absorption will serve as a useful working rule:TL = 20 log r + αr × 10 −3 (15.6)In Equation (15.6) the first term denotes the spherical spreading and the secondterm the absorption effect; with 10 −3 inserted to handle the fact that r is given inyards and α is expressed in dB/kyd.15.8 Underwater RefractionRefraction is the major factor in altering simple spherical spreading of sound inthe ocean. As mentioned above, the factors affecting the sound propagation speedin seawater are temperature, depth, and salinity. Variations of salinity do occur,particularly at the mouths of large rivers where copious amounts of fresh waterintermingle with seawater, at the edges of large ocean currents such as the GulfStream, and in water near the surface, where rain, evaporation, and ice melting canimpose maximal effects. Variations in speed of sound with depth are quite small