MELLEN: SOUND PROPAGATION IN A RANDOM MEDIUM 



Our technique was designed for simplicity of data analysis and 

 is represented pictorially in Figure 2. To eliminate boundary effects, 

 we made the experiments during the period when the surface tempera- 

 tures were sufficiently high to form a sound channel. SUS charges were 

 detonated on the channel axis as the transmitting ship opened range. 

 The signals were received by a hydrophone located on the channel axis 

 and recorded for later analysis. 



Analysis was accomplished by using 1/3-octave filters and 

 measuring the total received energy arriving through refractive 

 paths. This is no problem since arrivals reflected at the boundaries 

 can usually be separated in time. If not, they can be ignored since 

 they are more severely attenuated at least at the frequencies of 

 interest. The results are plotted in decibels (corrected for cylin- 

 drical spreading vs range) for each of the filter frequencies. An 

 example is shown in Figure 3. Then by linear regression analysis, 

 we obtain the attenuation coefficient. The validity of the cylindrical- 

 spreading approximation and the neglect of bottom loss above a critical 

 frequency were checked by DiNapoli (1971) in his Fast Field Program 

 and will be discussed later by Browning (in these Proceedings) . 



ATTENUATION EXPERIMENTS 



The saltwater results shown in Figure 4 together with earlier 

 work supported the conclusion of Thorp (1965) that the coefficients 

 below 1 kHz were anomalously high. The dashed line is the Marsh- 

 Schulkin curve that includes the MgSO relaxation absorption. The 

 excess absorption below 1 kHz is greater than predicted by roughly 

 a factor of 10. Thorp fitted the anomaly to a relaxation formula 

 with a relaxation frequency of 1 kHz. 



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