INTRODUCTION TO SONAR 



• Thermal noise: The absolute minimum noise 

 in the ocean is called thermal noise. As the 

 name implies, it is a function of temperature. 

 This noise is created by the motion of the 

 molecules of the liquid itself and is difficult 

 to measure. 



• Residual minimum: The lowest noise level 

 that normally is measurable is called residual 

 minimum. Different unknown phenomena un- 

 doubtedly contribute to the minimum observable 

 value. The point of interest, however, is that 

 you are never likely to encounter ambient noise 

 levels below the residual minimum. 



• Surface agitation: For a sizable region above 

 the residual minimum noise, the ambient noise 

 levels appear to be related to the surface agita- 

 tion. Usually, agitation of the surface of the 

 sea is measured as sea state. Surface windspeed, 

 however, ordinarily is a more reliable measure 

 of expected ambient noise than sea state. As 

 the wind rises, the surface becomes more and 

 more agitated, causing the ambient noise level 

 to rise. Normally noise levels in this region are 

 associated with what is called sea noise to 

 distinguish them from other noises, which really 

 are not caused by the sea itself. A heavy rain, 

 for instance, will add greatly to the ambient 

 noise. 



Flow Noise 



As an object moves through water, there 

 is a relative flow between the object and the 

 medium. This flow is easiest to understand by 

 assuming that the object is stationary and that 

 the water is moving past the object. If the 

 object is reasonably streamlined and its surface 

 is smooth, and if it is moving very slowly, a 

 flow pattern known as laminar flow is set up. 

 Such a pattern is shown in view A in figure 4-7, 

 where the lines represent the paths followed 

 by the water as it flows around the object. If 

 the flow is laminar, all lines axe smooth. Although 

 laminar flow produces little, if any noise, it 

 occurs only at very low speeds — perhaps less 

 than 1 or 2 knots. 



If the speed of the water is increased, whorls 

 and eddies begin to appear in the flow pattern, 

 as seen in part B in figure 4-7, and the phenom- 

 enon is called turbulent flow. Within these eddies 

 occur points where the pressure is widely dif- 

 ferent from the static pressure in the medium. 

 Thus we have, in effect, a noise field. If a 

 hydrophone is placed in such a region, violent 



71.22 

 Figure 4-7. — Patterns of flow noise. 

 A — Laminar flow; B — Turbulent flow. 



fluctuations of pressure will occur on its face, 

 and what is called flow noise will be observed 

 in the system. 



As pressures fluctuate violently at any one 

 point within the eddy, they also fluctuate violently 

 from point to point inside the eddy. Moreover, 

 at any given instant, the average pressure of the 

 eddy as a whole differs but slightly from the 

 static pressure. Thus, very little noise is radiated 

 outside the area of turbulence. Hence, although a 

 ship-mounted hydrophone may be in an intense 

 flow noise field, another hydrophone at some 

 distance from the ship may be unable to detect 

 the noise at all. Flow noise, then, is almost 

 exclusively a self-noise problem. 



Actually, not much information is known 

 about flow noise, but these general statements 

 may be made about its effect on a shipborne 

 hydrophone: (1) It is a function of speed with a 

 sharp threshold. At very low speeds there is 

 no observable flow noise. A slight increase 

 in speed changes the flow pattern from laminar 

 to turbulent, and strong flow noise is observed 

 immediately. Further increases in speed step 

 up the intensity of the noise. (2) It is essentially 

 a low-frequency noise. (3) It has very high 

 levels within the area of turbulence, but low 

 levels in the radiated field. In general, the 

 noise field is strongest at the surface of the 

 moving body, decreasing rapidly as you move 

 away from the surface. 



As the speed of the ship or object is in- 

 creased still further, the local pressure drops 

 low enough at some points to allow the forma- 

 tion of gas bubbles. This decrease in pressure 



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