E. Meyer 157 
reflection of shock waves of approximately equal pressures in air and water. 
Shock waves in gaseous media are a problem of nonlinear acoustics; while with 
the shock waves generated in water, it is probably correct to assume linear 
relations. This is also indicated by the respective Mach numbers (shock front 
velocity divided by velocity of sound) for equal pressures. 
DR. S. WENNERBERG pointed out that a possible way of checking the steep- 
ness of the shock front was to utilize the directivity of the receiving crystal. 
Assuming the rise time for the shock front to be 10-* sec, the velocity of 
propagation to be 1500 msec-!, and the diameter of the receiving face of the 
crystal to be 1 cm, then a rough calculation shows that the crystal has to be 
aligned to about 0.01 of a degree to obtain maximum response. He wondered 
whether experiments had been made to substantiate this. 
PROFESSOR MEYER: The question of Dr. Wennerberg is perfectly justified. 
The adjustment of the detector quartz is carried out in such a way that the in- 
dication on the CRT screen goes toa maximum. Indeed, very accurate adjustment 
is necessary. The detector quartz has a relatively large surface area, but only 
the size of the scanning electrode, on the rear side of the quartz, is of signifi- 
cance. The size of this electrode is about 2.5 mm. 
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