Sound- Pressure Level 



The behavior of the bubble produced by the pneumatic 

 source varies significantly from that produced by the under- 

 water spark. After rupture of the sphere, the resulting 

 bubble oscillates without complete collapse, until it finally 

 disintegrates. The pulse so generated remains at a single 

 frequency and does not contain the high frequencies that are 

 produced by chfjmical explosives or the underwater spark; 

 their bubbles collapse completely and re -expand during their 

 oscillation period, thus producing a broad band of sound 

 frequencies. 



In hundreds of measurements involving the 1-inch sphere 

 with 3/8-inch walls, the average peak sound-pressure level 

 for an omnidirectional condition was found to be 107 db/dyne/ 

 cm2 at 1 meter, at selectable frequencies within the band 

 25-250 c/s. This corresponds to an acoustic level of about 4000 

 watts, which is impressive at low frequencies. Much higher 

 levels appear obtainable in the near future. 



Waveform 



A typical waveform from a 1. 3-inch pneumatic sphere 

 is shown in figure 30. This form is obtained without any 

 filtering using an amplifier and hydrophone with a flat 

 response between 20 c/s and 10 kc/s. The sine wave at 

 the bottom of the figure is that of a 50-c/s calibrate signal. 

 It will be noted that the waveform has a very steep front, 

 probably indicating the presence of many frequencies, 

 and immediately goes into the low-frequency oscillation 

 which in this case is approximately 30 c/s. An unusual 

 effect is shown in figures 31 and 32. Here the rubber 

 material has ruptured in the form of a decayed sine wave 

 similar to the actual waveform (fig. 30). 



Pulse Length 



The acoustic signal generated by the bursting of the 

 pneumatic source is a damped wave of approximately 100 

 milliseconds of usable duration. This lends itself to 

 narrower band filtering in the receiver than can be used 

 with the much shorter pulse lengths generated by the 

 underwater spark source. 



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