Sound Navigation and Ranging, or SONAR, is a technology that uses sound to locate objects underwater.   First investigated by the British during WWI, by the beginning of WWII SONAR was used by submarines for navigation and target detection as well as a means for aircraft and boats to detect and target submarines.

While most are familiar with the iconic SONAR ping and receiving echo used in active SONAR, most early SONAR navigation and detection was done using a combination of active and passive SONAR.  Passive SONAR is a sound receiving system that uses hydrophones to detect underwater sound waves to indicate the presence, character, and movement of objects in the water without putting out any sound energy of its own.  

Not as accurate as active SONAR, passive SONAR held a distinct advantage for submarines and the boats searching for them.  Before the advent of nuclear propulsion systems, a submarine’s success in avoiding detection was determined by it’s acoustic signature.  A vessel using active SONAR and producing sound in order to detect its targets would then become a target itself by the production of that sound.  Using only the underwater ears of a passive SONAR hydrophone array to map the environment allowed submarines to run quiet and avoid detection.  

The burden of navigation and threat detection was placed upon the SONAR operator.  While visual interfaces could determine the location of an object detected by a passive SONAR hydrophone array, it was up to the SONAR operator to determine what that object was simply by listening.  It could be a whale, a merchant ship, an underwater mountain, or an enemy submarine.  Before a torpedo could be readied or a depth charge dropped, the crew had to be sure of their target.  This constant listening for hours on end was so demanding that US Naval SONAR operator on submarines and destroyers were limited to four hour shifts.  

This edition of Special Collections pulls its sounds from a collection of 78 RPM training records for US Naval SONAR operators published by the Naval Defense Research Center based on hydrophone recordings made at U.S.Navy Radio and Sound Laboratory in San Diego, California and U.S.Navy Underwater Sound Laboratory in New London, Connecticut.  Thanks to the Historic Naval Ships Association for the audio.