Chapter 4— PHYSICS OF SOUND 



represents the onset of cavitation. The noise 

 associated with this phenomenon differs from 

 flow noise. 



Cavitation 



Cavitation is produced whenever asolldobject 

 moves through the water at a speed great enough 

 to create air bubbles. After a short life, most 

 of the bubbles collapse. The sudden collapse of 

 the bubbles causes the acoustic signal known as 

 cavitation noise. Each bubble, as it collapses, 

 produces a sharp noise signal. 



Because the onset of cavitation is related to 

 the speed of the object, it is logical that cavita- 

 tion first appears at the tips of the propeller 

 blades, inasmuch as the speed of the blade tips 

 is considerably greater than the propeller hub. 

 This phenomenon, known as blade tip cavitation 

 is illustrated in figure 4-8. 



As the propeller speed increases, a greater 

 portion of the propeller's surface is moving 

 fast enough to cause cavitation, and thecavitating 

 area begins to move down the trailing edge of 

 the blade. As the speed increases further, the 

 entire back face of the blade commences cavi- 

 tating, producing what is known as sheet cavita- 

 tion. This form of cavitation is shown in figure 

 4-9, Cavitation noise is referred to as hydrophone 

 effects. 



The amplitude and frequency of cavitation 

 noise are affected considerably by changing 

 speeds. Cavitation noise versus speed at a 

 constant depth is diagramed in figure 4-10. 

 The curves shown are idealized; they do not 



FREQUENCY IHERTZI 

 500 1000 5000 10.000 



170 RPM 

 150 RPM 

 30 RPM 

 1 1 RPM 



Figure 4-9. 



71.24 

 ■Sheet cavitation. 



represent any particular submarine. Note the 

 great difference in noise level caused by the 

 addition of only 20 turns, from 90 rpm to 110 

 rpm. At 90 rpm there is no cavitation; the noise 

 level is due to flow noise and background noise. 

 At 110 rpm cavitation has started, and the noise 

 level has gone up many decibels, with a peak 

 amplitude at about 400 Hz. As speed is increased 

 further, the amplitude peak tends to move toward 

 the lower frequencies. The amplitude increases 

 at a lesser rate, but covers a broader frequency 

 band. The curves are also characteristic of a 

 submarine on the surface and of surface war- 

 sliips, although the amplitude levels may be 

 different. 



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WATER FLOW 





WATER FLOW' 



71.23 

 Figure 4-8. — Blade tip cavitation. 



35.50 

 Figure 4-10. — Speed effect on cavitation noise. 



41 



