490 



VIGOTJREUX AND HEESEY 



[chap. 12 



not clear why water noise should be relatively richer in high-frequency 

 components. 



The noise spectrum illustrated in Fig. 5 is modified in coastal waters where 

 waves breaking on the shore and local currents bring their contribution; 

 nevertheless the change of character is surprisingly small. A much more 

 noticeable change is produced by animal life in some coastal regions where the 

 water temperature is 15°C or above. Small fish called croakers can cause as 

 much noise again as sea noise of state 1, at frequencies below 2 kc/s, and 

 snapping shrimp will readily produce a similar increase at frequencies up to 

 20 kc/s or more. Curves given by Horton (1957) are reproduced in Fig. 5; the 

 shapes apply to the calmest sea conditions, but the volume, of course, depends on 

 the number of the congregation . Since World War II biologists have learned much 

 more about the characteristic sounds of many soniferous marine animals. These 

 findings are discussed in Chapter 14, "Sound Production by Marine Animals". 



-80 



Euj 

 ^ o 



> <j 



o ^ 



< Q 



z 



(/) < 



-I m 



LlJ 



OQ < 



-110 



-140 



■170 



200 



Fig. 



2 20 



FREQUENCY, kc/s 

 5. Spectrum level of water noise and of croaker and shrimp. (Reproduced by per- 



mission from Fundamentals of Sonar by J. W. Horton. Copyright 

 Nav'al Institute, Annapolis, Md.) 



1957 by the U.S. 



The third cause of noise is man-made, and the most important and interesting 

 in this class is that due to cavitation at propellers of ships. The driving thrust of 

 a propeller blade is produced by circulation of water round the blade, which 

 reduces the pressure on the forward face and forms vortices at the edge. 

 Cavities form when the pressure vanishes or a little before, and their subsequent 

 collapse on the blade or on themselves gives rise to cavitation noise. At high 

 frequencies the spectrum level decreases by 6 dB per octave according to 

 expectation, and below 1000 c/s the slope decreases also in general qualitative 

 agreement with theory, but the shape varies from ship to ship, and with speed 

 for any one ship. 



The noise of propellers is useful only when it is emitted by a ship one is 

 trying to detect. In all other cases it is regrettable, but is almost inevitable in 

 propulsion other than by sail. It interferes with the sonar set of the ship, 

 limiting the range of detection, and even though it may help detect another 

 ship, it masks the echo somewhat, making echo ranging more difficult. 



