2.01 



1.0 



•05 



u 



r 



U T 400 cm/sec. 

 Q, 0.24 cm 



Initial contact 



_ut 



a 



Figure 17. Pressure pulse produced by the impact of a water droplet. 



part of a high-speed motion picture which Franz made prior to his investigation; the 

 appended oscillographic record does not correspond to the particular splash appearing 

 in the photographs but shows, nevertheless, the considerable time interval between the 

 impact sound and the bubble sound. Attention is directed to the fact that no measur- 

 able sound is produced during the greater part of the interval in which the fascinatingly 

 varied undulations of the surface occur. 



Franz also measured the underwater noise generated by steady showers of 

 water droplets falling on an extended area. The results agreed with the measurements 

 of the sound made by solitary droplets and, so far as available meteorological data 

 permit comparison, with underwater noise levels measured during rainfall. (The 

 sounds from a continuous succession of randomly spaced impacts add incoherently 

 so that the total energy in each band of frequencies is conserved). 



Similar, though less extensive, data were obtained for the entry of solid objects 

 of various shapes. An intriguing theoretical problem is that of relating the time- 

 dependent doublet strength, and hence the sound field, to the shape and motion of 

 the entering body, or to its shape, mass, and entering velocity. It does not appear 

 likely that the answer will be obtained in any simple form. A special case is the entry 

 of a massive solid body having a conical nose. Here, considerations of similarity indi- 

 cate that the sound pressure pulse must begin as a "ramp function", i.e. must begin as 

 zero and increase linearly with the time. 



The preceding discussion has emphasized the gaps in available concrete informa- 

 tion concerning sound produced by surface disturbances. Perhaps, however, its pres- 



263 



