450 



32 



Slope equals Initial 

 Impulsive Velocity ■ 



Figure 2/Va - An Idealized Displacement-Time Curve 

 of the Center of a Diaphragm of the U-Class 



The existence of the pause at time („ is uncertain. 



::Z 



Slope equals Initial 

 Impulsive Velocity- 



Figure 22^h - An Idealized Displacement-Time Curve 

 of the Center of a Diaphragm of the L-Class 



Figure 2U - Typical Displacement-Time Curves of Center Spots 



on the Diaphragms 



one-half to two-thirds of the cases up to time (j the overall displacement 

 is about what it would have been if the jerking motion had not been present. 

 At time t , the bending wave reaches the center, as illustrated in Figure 23, 

 which then comes to rest or undergoes a slight elastic return. 



There appear to be two different modes of vibration in the dia- 

 phragms. This divides the diaphragms into two classes corresponding to these 

 modes of vibration. In the first class, referred to as the upper or U-class, 

 the diaphragms regain at the time <, the entire displacement lost during the 

 pauses, as shown in Figure 2Ua. In the other class, referred to as the lower 

 or L-class, the lost deflection is not fully regained as in Figure 2Ub, and 

 the deflection at the center is less than if the motion had continued at a 

 uniform rate. The origin of the Jerking motion in the flat central region 

 is not known, nor is it known why there seem to be two different modes of 

 this type. One suggestion* is that the jerking is due to a pressure wave re- 

 flected back and forth between the central area of the diaphragm and a cavi- 

 tated region in the water. In other words it may be caused by vibrations in 

 a layer of water left on the undersurface of the diaphragm as it is displaced. 



Figures 25, 26, and 27 show the variation with charge distance of 

 the measured times t^', t ^" , and ij respectively. The data on t^ were too 

 meager and difficult to measure to indicate any significant variation with 

 distance. The measurements which were made, however, give an average for t^ 

 of about 0.12 millisecond. The time t ^' decreases slightly from 0.2 milli- 

 second at a charge distance of 8 inches to about 0.1 6 millisecond at a charge 

 distance of 13 inches. At greater charge distances this value remains prac- 

 tically unchanged. 



This suggestion was first made to the writer in a discussion with Dr. P. Fye of the Woods Hole 

 Oceanographic Institution. 



