TRANSMISSION RUNS 



73 



HORIZONTAL PLANE 



VERTICAL PLANE 



Figure 3. Directivity patterns of the JK SK4926 at 24 kc. 



strictive) and JK (X-cut Rochelle salt), the direc- 

 tivity index is approximately —23 db at 24 kc. 



The total power output of a projector is usually of 

 less interest than the power output per unit solid 

 angle on the axis. This quantity is customarily re- 

 ported in terms of the source level <S, which has al- 

 ready been defined in Section 4.1. The source level of 

 the gear used in the UCDWR transmission studies is 

 about 107 db above 1 dyne per sq cm 1 yd from the 

 projector face. 



The receiving instruments in transmission runs are 

 usually cable hydrophones. The sound head consists 

 of the electroacoustical element itself and sometimes 

 contains a preamplifier which boosts the output 

 voltage before it passes through the cable to the main 

 sound stack. The electroacoustical element itself 

 may be either a crystal element (as in the CN8 series 

 used for a long time at UCDWR), or it may be a 

 magnetostrictive device (similar to the Harvard 

 Underwater Sound Laboratory [HUSL] B19-H). 



The receiving response of a hydrophone is defined 

 as the ratio between the rms voltage across the out- 

 put terminals of the hydrophone or the preampHfier 

 and the soimd pressure of a plane wave incident on 

 the axis of the hydrophone. It is ordinarily reported 

 in decibels above 1 volt per unit sound pressure and 

 then denoted by s. Waves incident in directions not 



270 



180° 

 Figure 4. Response pattern of the CN-8-2 No. 597 

 hydrophone at 24 kc in horizontal plane. 



parallel to the axis will produce lower voltages than 

 sound waves of the same amplitude incident on the 

 axis of the hydrophone; in other words, most hydro- 

 phones discriminate against off-axis sound inputs. 



