a 740-ton submarine has a length of approximately 244 feet» 

 There are large areas of the submarine, like the conning 

 tower, which are good reflectors of sound <, 



These two characteristics of the submarine--that it 

 creates noise and reflects sound—complement a characteristic 

 of sea water— that it is an excellent medium for the transmis- 

 sion of sound—far better than airo 



Difficulties of Underwater Sound Detection. 



But the business of transmitting and receiving sound un- 

 der water is not a simple one Particularly is this true of 

 echo-ranging Many other objects will reflect sound, includ- 

 ing whales, schools of fish, patches of sea weed, the wake 

 of another vessel, the frothy zone on the edge of a tidal 

 current, and rocks and wrecks on the bottom 



When echo-ranging j, it is not easy to get an echo in the 

 first place or thereafter to keep contact with an elusive 

 target like a hunted submarine When a sudden sound is made 

 under water, the ocean behaves very much like a large empty 

 room with bad acoustic properties The sound echoes and re- 

 echoes 9 so that it is difficult to hear any small sounds such 

 as a returning echo from a submarine Especially in shallow 

 water or in a rough sea, the reverberation may be so intense 

 as to drown out the echo entirely Underwater sounds from 

 the motion of the ranging ship create a disturbance much like 

 static on a radio c 



Moreover, as will be seen, sound does not always travel 

 in a straight line but is refracted due to changes in water 

 temperature with depth which cause differences in water den- 

 sity and consequent changes in sound velocity,, These temper- 

 ature gradients bend the sound waves Lastly, operators who 

 have spent days in an empty ocean straining their ears for 

 an echo, may become "ping happy" and be unable to detect an 

 echo should one come in D 



The improved German submarines of World War II could 

 crash dive at approximately 2 feet per second and from peri- 

 scope depth could reach 200 feet in approximately 90 seconds . 

 They could dive to a depth of 600 feet or more making a 90- 

 degree turn at 4 knots in slightly more than 2 minutes, so 

 that 100 seconds after a submarine was seen at a given spot 



22 



