SECT. 4] SOUND IN THE SEA 493 



C. Explosive Charges 



Explosive charges are convenient sources of sound for the study of propaga- 

 tion, not only because of their simplicity but also because the spectrum of the 

 explosive pulse extends over the whole range of frequencies which are of 

 interest, at any rate for long-range propagation. 



The pressure at the time and place of explosion is many times the static 

 pressure, and gives rise to a shock w^ave. The pressure of this wave decreases 

 with range more rapidly than according to geometric spread and, therefore, 

 from energy considerations, the time constant, which would be independent of 

 range for an acoustic wave, increases wdth range. But at the long ranges which 

 concern us here the disturbance becomes \\eak and travels as an acoustic wave ; 

 the spectrum is still very wide although its shape is gradually changed by the 

 greater absorption of high frequencies. 



The law of propagation for any frequency is studied by firing charges at 

 various ranges and observing the energy in a known frequency band, as ex- 

 plained in the previous section. In practice it is usual to cover the whole available 

 spectrum simultaneously by contiguous filters and associated recorders. 



Although a knowledge of the spectrum level of explosive charges is not 

 essential to all studies of propagation, it is of great help in planning the in- 

 vestigation. The curve of the energy spectrum level is not perfectly smooth 

 because of the oscillation of the explosion bubble and of surface effects, but 

 there is a general rise to a maximum, at about 15-20 c/s for a 1 lb charge, then 

 a fall at an increasing rate which eventually becomes constant at 6 dB per 

 octave, the terminal slope expected from a sudden rise of pressure followed by 

 an exponential fall. Weston (1960) has measured the energy spectrum level of 

 a 1 lb charge of T.N.T. fired 60 fathoms below the surface in deep water. His 

 results are reproduced in Table I. 



Table I 



Spectrum Level of Energy Flux Density for a 1 lb Charge of T.N.T, Fired 

 60 fathoms below the Surface, in dB above 1 joule/m^ in a band 1 c/s wide at 



100 yards 



For a charge of W lb the frequencies must be divided by Tf'/^ and the spectrum 

 levels increased by (40/3) log W . 



Weston has also discussed the change of spectrum level with depth of 

 charge : for the 1 lb charge a correction is required at 35 and 70 c/s, but at 

 frequencies above 100 c/s the change is too small to matter. 



