22 



Direction of 



^ 



Incident 



Pressure Wove 



E 



High 



Pressure 



Figure 6 - Diagram to Illustrate 

 Diffraction of a Pressure Wave 



By moving forward, the diaphragm relieves the 

 pressure, and eqiwlization of the pressure by 

 diffraction then occurs in the direction of 

 the arrows E. 



diaphragm mounted In a heavy ring, be- 

 cause the forward motion of the dia- 

 phragm relieves the pressure over the 

 diaphragm, a process of equalization of 

 pressure In the water sets In and acts 

 to lower the pressure In front of the 

 ring and to raise It in front of the 

 diaphragm, as Illustrated In Figure 6. 



Since, however, effects of 

 moderate magnitude are propagated 

 through water only at the speed of 

 sound, the equalization requires time 

 for Its completion. Thus, during an 

 Interval much shorter than the diffrac- 

 tion time, after a shock wave has 

 struck a plate, lateral equalization 

 of pressure between the water in front 

 of the plate and that beyond its edge, or even between different parts of the 

 plate, will not have had time to progress very far. During this short time 

 each part of the plate will respond to the incident wave more or less inde- 

 pendently, according to the laws that hold for the one-dlraenslonal action of 

 shock waves on plates. 



In the example Just described. If the plate is 10 feet across, the 

 diffraction time T^ is one millisecond. This exceeds the compliance time T„ 

 by a good margin for plates up to a thickness of 1 or 2 Inches, as Is evident 

 from Table 1, so that the one-dimensional formulas should give good results. 

 On a plate 10 Inches thick, however, diffraction from the edge would produce 

 a large effect. 



It has been assumed in the foregoing discussion that appreciable 

 stress forces are not called into play by the small displacement of the plate 

 that occurs during the time T„. This is usually true in practical cases. In 

 the example just described, for instance, the maximum velocity acquired by a 

 1-inch plate is, from Equation [71. 



2100 



2 ^ 5.5'*-'= 510 m/sec 



Even in a millisecond, therefore, the plate will have become displaced by 

 less than half of its thickness. Stress forces, if appreciable, would have 

 the effect of reducing the maximum velocity. 



Phenomena in the second phase of the action, now to be discussed, 

 will depend upon whether cavitation does or does not occur. 



