Sound-Pressure Measurements 



The sound-pressure levels listed below were measured 

 at the NEL Pend Oreille Calibration Station in Idaho in 

 July 1959. The lake is 750 feet deep at this point. (Photo- 

 graphs of bubble development at NEL POCS appear in 

 figure 14. ) 



Using an energy source of 3000 watt- seconds, a peak, 

 broadband sound- pressure level of 135.2 db/dyne/cni^ 

 referred to 1 meter was measured with the underwater 

 spark source 100 feet below the surface. This was for a 

 nondirectional condition and corresponds to a peak acoustic 

 power in the water of about 3 million watts. 



Additional omnidirectional measurements, made at a 

 depth of 300 feet and using low- pass filtering, show the 

 following results: 



Low- pass 



Acoustic Pressure Level 

 (db/ dyne/ cm ref. 1 meter) 



Approximate 



Cut-off (c/s) 



Acoustic Level (kw) 



200 



114.0 



25 



1000 



122.0 



150 



4000 



122.0 



150 



5000 



122. 5 



160 



As can be seen, a substantial amount of energy appeared 

 in the region between 1000 and 4000 c/s. Reference to the 

 graph in figure 5 will show that the acoustic level in the 1000 

 to 4000 c/s region is 300, 000 watts. There are few, if any, 

 existing sound sources generating powers of this magnitude. 

 Since the source levels were for the nondirectional case, it 

 appears that additional gains can be made by operating the 

 spark source at the focus of a reflector. The dimension of 

 such a reflector at this frequency is practical for many 

 applications where directivity is desired. 



Since these measurements were made, the electrical 

 storage- transmitter power has been doubled. However, no 

 sound- level measurements have been made using the new 

 storage unit. 



EXPERIMENTS WITH A MAGNETIC GAP 



A few experiments were conducted to determine the effects 

 of a magnetic field on the spark gap. The first simple measure- 

 ments used a spark plug inserted between the poles of a large 

 magnetron magnet (fig. 15). Later experiments employed 



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