STUDY OF FREE-RANGING SHARKS 461 



Pulse rate— When pulse rates are slowed to less than about 1/s, 

 tracking also becomes increasingly difficult, especially in noisy areas. 

 Tracking personnel distinguish transmitter pulses from noise pulses mainly 

 on the basis of the repetitive pattern in which they occur. Hearing just one 

 or two pulses does not confirm signal presence, except for pulses long 

 enough to possess an unmistakable tonal quality. The tracker requires at 

 least several successive pulses at the expected rate before recognition can be 

 confirmed. At excessively slow pulse rates, the listening time needed for 

 recognition becomes too long. This can be a serious disadvantage in reef 

 areas where transmission is often interrupted as the animal moves behind 

 signal-blocking structures. Similarly, a slow pulse rate necessitates a slow rate 

 of scanning with the directional hydrophone so that it takes much longer to 

 complete a 360° scan of the underwater horizon. In general, pingers or 

 single-channel USTs are easier to recognize than multiplexed units with more 

 irregular pulse intervals. With the latter, it is even more important that pulse 

 intervals not be excessively long. 



Receiver Characteristics— When considering the effect of the receiver/ 

 hydrophone system on theoretical signal-detection range, the following 

 characteristics are pertinent. 



Absolute sensitivity— For a signal in the receiver headphones to be 

 recognized by the tracker, it must be present at the receiver input above a 

 certain absolute level; otherwise it will be lost in the receiver's internal noise. 

 This value is usually called the receiver sensitivity or receiver noise level and 

 is specified in microvolts, e.g., the DuKane N15A235A receiver noise level is 

 given as 0.1 juV equivalent at input. Thus, to be recognized, the electrical 

 signal from the hydrophone must be somewhat above 0.1 juV— no matter 

 how low the environmental noise might be. 



This shifts the question to what level of acoustic signal at the hydrophone 

 is needed to produce the required 0.1 uV electrical output. Hydrophone 

 sensitivity is usually given in dB re 1 V/juPa, i.e., the voltage (in terms of dB 

 below 1 V) that results from a sound pressure of 1 uPa. Again, using the 

 DuKane receiver as an example, its hydrophone is specified as being a mini- 

 mum of —185 dB re 1 V/uPa. From Table 5, it is seen that 0.1 //V is equiva- 

 lent to —140 dB re 1 V. Therefore, any acoustic signal of less than 45 dB 

 re 1 /uPa cannot be detected because it will not produce the required 0.1 uV 

 at the receiver input (45 dB above —185 dB = —140 dB). 



Bandwidth— An important characteristic of the receiver /hydrophone 

 system is the effective frequency bandwidth, usually defined as the total 

 width (in hertz) to the half-power points (3 dB down) on both sides of the 

 peak of the frequency-response curve. For the DuKane receiver, bandwidth 

 to the —3 dB points appears to be considerably less than 1 kHz (±500 Hz). 6 

 In general, a relatively narrow band receiver is preferable because the 

 narrower the bandwidth, the more the ambient noise that is excluded. This 



Specifications state a 1-kHz bandwidth to the —10 dB points. 



