SHOCK WAVE MEASUREMENTS 



267 



to the extent that it is Hnear (i.e., of small amplitude) be regarded as a 

 superposition of sinusoidal waves of suitable frequency components 

 corresponding to the steeper portions of the pressure-time curves. 

 These frequencies are attenuated more rapidly than the lower ones, and 

 one should therefore expect a progressive rounding of the initial peak 

 of the shock wave, which however -becomes significant only over dis- 



\ SOURCE / / 



1/K 



/ / 



/ 



/ 



/ 



/ 



/ 



/ 



/ 



/ 



/ 



Fig. 7.22 Refraction of a sound wave as a result of sound velocity decreasing 



with depth. 



tances of hundreds of yards for charges of reasonable size. The exact 

 analysis of this absorption effect is complicated by a necessarily co- 

 existing dispersion, or dependence of sound velocity of a continuous 

 wave on frequency. Ideally, measurements of transient explosion 

 pressures offer an excellent means of examining such effects, but instru- 

 mental and environmental difficulties in such measurements have not 

 as yet permitted their satisfactory realization. 



B. Refraction effects. The propagation of shock waves outward 

 from an explosive source may be greatly modified by variations in the 

 medium through which it travels, as well as by the presence of bound- 

 ary surfaces. The effect of the medium, which results from variations 

 in the velocity of propagation at different points, is under any practical 

 conditions negligible for distances at which any ordinary explosion wave 

 is sufficiently intense to inflict damage. At distances exceeding a few 



