Now let's see what is required to increase 

 a sonar's range. To double the range of a sonar 

 requires a source level increase of approximately 

 27 db. This power increase is equivalent to about 

 500 times, which obviously is impractical. By 

 another method, such as increasing receiver 

 sensitivity, the range can be increased fairly 

 easily, although it may not be doubled. 



So far we have discussed the decibel mainly 

 in relation to power output, but the decibel also 

 is used to determine receiver sensitivity. Re- 

 ceiver sensitivity is measured in minus numbers, 

 which represent the number of decibels below a 

 reference level that a signal can be detected by 

 the receiver. The larger the negative number, 

 the better the sensitivity. Although the range 

 would not be doubled, it is more practical, for 

 instance, to increase a receiver's sensitivity 

 from 112 db to 115 db than it is to double the 

 output power of the sonar transmitter. Also, 

 it is not always necessary to increase the 

 sensitivity of the receiver itself. An effective 

 increase of several decibels may be achieved 

 if local noise levels can be reduced in such 

 sources as machinery, flow noise, crew noise, 

 etc. 



NOISE 



The most complex sound wave is one in 

 which the sound consists of numerous frequencies 



71.116 

 Figure 4-6, — Noise sources. 



across a wide band. Such a form of sound is 

 called noise because it has no tonal quality. 

 The source of several types of noises may be 

 identified easily, however, because of the standard 

 sound patterns of the noises. Ship noise, for 

 instance, consists of many different sounds mixed 

 together. You may be unable to distinguish a 

 particular sound, but, on the whole you could 

 recognize the sound source as a ship. Some 

 noise sources are shown in figure 4-6. The 

 sources of many noises detected by sonar have 

 not yet been identified. Explanations of some 

 kinds of noises that have an adverse effect on 

 a sonar operator follow. 



Ambient Noise 



Ambient noise is background noise in the sea 

 that is due to natural causes. Different phenomena 

 contribute to the ambient background noise. 

 Many of the sources are known and their effects 

 are predictable, but many still are unknown. 

 From this unwanted background noise, the sonar 

 operator must be able to separate weak or 

 intermittent target noises. In general, the average 

 operator requires a target signal strength of 

 4 db above background noise. 



39 



