152 Lecture 9 
Fig. 9.19. Schlieren shad- 
ow picture of shock wave 
in water. 
kK +7? cm 
The physical effects are illustrated with a number of oscillegrams recorded 
with the electrodynamic shock wave transducer of W. Eisenmenger. A copper 
membrane terminates a water column as described in Section 9.1.5, through 
which the shock wave generated by the impact of the membrane propagates. The 
sound pressure is measured with a small piezoelectric hydrophone described 
in Section 9.2. Figure 9.18 demonstrates the formation of the shock front with 
increasing distance from the transducer (pressure, 75 atm). The lower diagram 
displays the plot of the current in the sound generator. 
A Schlieren photograph of the shock wave is presented in Fig. 9.19. When 
such a shock wave hits the rear endof the water column with a pressure-release 
termination, it is reflected with phase reversal and the resulting high under- 
pressure causes strong cavitation in the entire water column like the unbottling 
of soda water, 
Now the question arises, how to measure the rise time of the pressure or 
the thickness of the shock front more exactly. For this purpose, the quartz 
coaxial line microphone described in Section 9.2 is the proper instrument be- 
cause it provides the ultimate resolution with respect to time. The oscillogram 
presented in Fig. 9.20 was obtained from the quartz coaxial line microphone 
with a laboratory oscilloscope with maximum sweep magnification. The front 
surface of the quartz rod is exactly parallel to the incident shock front. The 
shock wave is repeatedly reflected inside the quartz as shown in Fig. 9.20b. 
It is inferred from such oscillograms that the shock process lasts less than 
10-8 sec, so the bandwidth of the oscilloscope is obviously insufficient (upper 
Fig. 9.20. Shock front oscillogram 
with quartz hydrophone. 
t+ Olu sec 4 Siu sec 
(a) (b) 
