C/W Robertson
    C/W Watson.
Top Secret.   Copy No. 43.


  Current Order No. 23 - August 1944  
  Density layers of Water and listening conditions.   Investigations concerning the salt content and temperatures of density layers in the areas of open sea have proved that layers of water of different temperature and different salt content circulate and displace each other.  Since the acoustic range is strongly influenced by raising and falling temperatures, it is continually changing.  
  As Result of Investigations.  
1. Measure density of water, with boat in action.
2. If listening reception seems poor, vary diving depth higher or lower, to obtain better signal strength.  Also alteration of diving depth downward can improve listening conditions.
3. Since known under water hydrophones work through measurement of sound, in case of pursuit, work with density measurement and supervision of the signal strength of the acoustic interception.  Go into water layers unfavorable for sound transmission.



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