MELLEN: SOUND PROPAGATION IN A RANDOM MEDIUM 



the angular distribution of normally trapped rays increases so that 

 energy is continuously lost from the channel. The sound channel 

 attenuation model includes absorption, diffusion loss (Mellen et al., 

 1974) , and diffraction loss. Here the diffraction component is for 

 the first normal mode only and assumes an infinitely lossy bottom. 



These data can be fitted empirically, as -shown in Figure 9, by 

 adding an extra loss independent of frequency. For the Hudson Bay 

 experiment, the excess is 0.04 dB/kyd while for the Gulf of Aden 

 (Browning et al., 1973) value, it is 0.02 dB/kyd. The MgSO component 

 was corrected for temperature, -1.5°C for Hudson Bay and +15°C for 

 the Gulf of Aden. The most significant difference between the two 

 experiments is that the Thorp coefficient for the Gulf of Aden is 

 only 0.6 that for Hudson Bay, which suggests differences in boron 

 chemistry of the two bodies of water. 



Once the possibility of a constant diffusion loss independent 

 of frequency was accepted, the results of Lake Superior (Browning 

 et al., 1968), shown in Figure 10, became clear. At first we had 

 guessed that the Thorp relaxation was common to both salt- and fresh 

 water, with only the MgSO component missing in Lake Superior. It 

 was later found that the necessary boron content did not exist in 

 Superior which gave strong support to the scattering hypothesis. 



We have used the term "independent of frequency" to describe 



diffusion which is, of course, a large ka approximation. For 



o 



ka <<1, we expect the loss to fall off. There may be a hint of 

 o 



reduced scatter at 530 Hz which would make the scale size a = 0.5 m. 



o 



Further support to the scattering hypothesis was given by the 

 experiments in the South Pacific (Bannister, 1976) which show an 

 excess absorption of 0.002 dB/kyd (see Figure 11). Like the North 



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