15 
For any point on the sea surface, 
r =r' and equation 2) gives zero 
pressure corresponding to the 
condition that the pressure at this 
surface remains unchanged by the 
pressure pulse. 
42, For a point P below the sea 
surface, r’>r, and the pressure 
given by equation 24 will be the 
difference between the two curves 
OC D and O'C'D' in figure 6a. 
These curves are essentially of the 
same shape but differ in magnitude 
by the ratio r/r'. The time 
difference 00' = (r' - r)/c 
corresponds to the longer time taken 
for the reflected pulse to travel 
effectively from E', as compared 
with the time taken by the original 
pulse to travel from E. The 
resultant pressure at P given by 
equation 2} is then of the form 
0C 0O' C' F in fig. 6b. 
Fige 5 - The reflection of the 
ressure pulse at an 43e Such use of equation 2) leads 
air/water surface to negative pressures in the water 
which could be of much the same 
order as the maximum pressure in the original pulse. Some qualification 
is obviously necessary to allow for the fact that water cannot, in general, 
withstand large tensions. Experimental evidence on the tensile strength 
of water is somewhat contradictory. Under laboratory conditions, with all 
air bubbles removed as far as possible, ordinary water can apparently 
withstand static tensions of the order of 500 lb. per sq. in, whilst water 
nearly saturated with air can withstand static tensions to about 80 lb. per 
Sqe in. Under dynamic conditions, the strength is probably less and drops 
to an almost negligible value if the flow becomes turbulent. 
00's (rr) OK=@Lr) 
c c 
PRESSURE 
PRESSURE 
(2) (b) 
Pig. 6 - Pressure/time variations for a point below the sea 
surface 
