released, the air bubble alternately expands and contracts, 

 providing a wave train at the resonant frequency of the 

 bubble. The frequency of a given bubble size is controlled 

 by the stiffness of the medium, which is proportional to 

 depth. Thus, the frequency of oscillation can be controlled. 

 Preliminary tests with this source were encouraging. 

 Work has continued, and thousands of pneumatic explosions 

 have been made with uniform results. Frequencies between 

 5 and 300 c/s have been generated in this fashion. 



EARLY EXPERIMENTS 



The early trials consisted of inflating many types and 

 shapes of elastic material underwater. Surgical rubber 

 tubing was tried in many lengths and wall thicknesses. 

 Rubber bulbs, plastic bottles, and glass beakers were also 

 tested and were blown to destruction. Best results were 

 obtained with surgical rubber because of its uniformity. 

 For inflation, both compressed air and carbon dioxide were 

 used- -the latter in the form of carbon dioxide cylinders, 

 with their triggering mechanisms, from standard Navy life 

 jackets. Figure 22 shows one of the early designs. 



^Hf"*^ 



Figure 22. Pneumatic high- power/If sound source, with 

 6-8 gram CO„ cartridge. 



Novel Low Frequency So u nd Sources , by L. R. Padberg, Jr. 

 NEL Lett"er Report 38, CONFIDENTIAL, 14 October 1957 



43 



