STUDY OF FREE-RANGING SHARKS 



459 



high-level spikes can drive the meter reading up to a point from which it 

 cannot subside sufficiently in the relatively quiet periods in between. When 

 tracking in reef areas, it is generally preferable to position the boat so that 

 the hydrophone need not be pointed in the general direction of noisy reef 

 surfaces that are much closer than the transmitter being tracked. 



Transmitter Characteristics 



Frequency— It is evident from Tables 2 and 3 that at very short 

 distances significant transmission losses are almost entirely due to spreading, 

 whereas at relatively long distances the losses become increasingly due to 

 absorption (which is frequency dependent). From this, it follows that, if 

 relatively long ranges in sea water are desired, frequency becomes a critical 

 parameter and it becomes more important to use a lower frequency. 

 Considering that the higher ambient noise at lower frequencies partially 

 negates the lesser absorption at lower frequencies, it is possible to calculate 

 for any given distance which frequency will theoretically reach that distance 

 most efficiently. From Figure 22 it can be seen that, if a maximum range of 

 only 200 m is desired, the optimum frequency is much higher than if a range 

 of 2 km is attempted. 



Output power— Since transmitter range is partly a function of 

 transmitter signal strength, the question arises as to how much electrical 

 power is required to achieve a given sound pressure level (SPL). The 

 significant factors are (1) the efficiency of the transmitter power-amplifier 

 section in converting battery power into higher voltage power at the input to 



CE 



UJ 



O 



Q- 





200 r 



175 



50 - 



=L 125 - 



2 2 

 £~ 100 



< 



75 - 



50 



100 150 200 



FREQUENCY (kHz) 



250 



300 



Figure 22 Transmitter strengths required to achieve various ranges in coastal 

 sea water (sea state 3) as a function of frequency. For a signal-to-noise ratio 

 of dB (RD = 0), a receiver bandwidth of 1 kHz, and a moderately direc- 

 tional hydrophone. Redrawn from Stasko and Pincock (1977). 



