10 
3 
Empirical data for the pressure pulse 
22. The theory of small-amplitude sound pulses is sufficient to 
describe the propagation of the pressure pulse except in the immediate 
neighbourhood of the charge. For aT.N.T. charge, the maximum pressure 
in the pulse becomes of order 2 tons per sqe ine or less at distances 
beyond about 12 charge diameters from the explosion. However, this 
simple theory gives no indication of the shape of the pulse, that is the 
form of the function f in equation 1, since this shape depends on the 
course of events in the inner region round the charge. Nevertheless, 
for distances at which the pressure pulse behaves to a reasonable 
approximation as a sound pulse of smali amplitude, the simple theory can 
be used in conjunction with experimental evidence to deduce the shape of 
the pulsee 
23. Fige4h is a typical 
experimental record, obtained by 
a tourmaline gauge and shows the 
variation of pressure with time 
in the pulse. The curve 
indicates an initial vertical 
front, corresponding to a sharp 
3000 
2 instantaneous rise of pressure 
¥ 2000 to a maximum value, followed by 
g decressing pressuree Unlike 
é blast in air, the pressure 
w remains positive throughout with 
FA no evidence of any subsequent 
= ae appreciable suction phase. 
Empirical analysis of experimental 
results indicates that the initial 
decreasing portion following the 
front is exponential in shepe, 
but that in the final "tail" the 
° pressure decays more slowly than 
predicted by the exponential 
as acl moos ai curve which fits the initial 
TIME - sec. portion of the empirical curve. 
However, it seems fairly certain 
that the final tail is relatively 
Fige4. - Empirical curve showing the unimportant so far as the 
pressure/time variation damaging power of the pulse is 
concerned. Therefore, it is 
usual for theoretical analysis of damage to assume the pressure/time curve 
to be of the exponential form 
p = pe ‘gc cob ‘dda, ooo \ food | iobo wdog © Gn (10) 
where p,, is the maximum pressure in the pulse, n deterniines tiie rate of 
decay of pressure, and t’ is time measured from the arrival of the pulse at 
any point under consideration. 
2h... It is found that for distances beyond which pp, is of the order of 2 tons 
per Sqein. or less, measurements are in reasonable agreement with the 
preceding acoustic theory which predicts that p, should vary inversely as 
the distance and that n should be independent of distance. Enpirical 
formulae for pm, n and related quantities have been prcposed from data 
derived from underwater experiments with the large type tourmaline strip 
geuge. With such a recording technique, measurejents were necessarily 
confined to distances greater than 100, times the charge radius. Hewever, in 
recent years, an experimental techniqué ‘hich devends on amplification of 
Signals from very much smaller and simpler tourmaline gauges has been 
developed and it is now possible to investigate underwater effects at 
exceedingly small ranzes. Using this technique, data for pressure, momentum 
and energy have been obtained for the whole range of distances inciuding 
