STUDY OF FREE-RANGING SHARKS 465 



neglects losses from the more unpredictable factors, such as scattering and 

 refraction, the actual transmitter range will probably be less. Even shorter 

 ranges would be expected at higher ambient noise levels, or when the re- 

 ceiver is not in the optimum radial direction from the transducer element 

 of the transmitter. It must be kept in mind that the previous discussion is 

 intended only as a simplified introduction to the kinds of numbers involved 

 in calculations of range estimates and should not be regarded as a method 

 for precise prediction. Before any reliance is placed on calculated estimates, 

 in situ verifications should be made. Range calculations confirmed by actual 

 measurements in coastal seawater are graphed by Stasko and Pincock (1977) 

 for various UST frequencies (Figure 22). 



Transmitter Size, Life— Although much of the transmitter circuitry 

 can be miniaturized to very small dimensions, overall package smallness is 

 still very much limited by battery size and, to a certain extent, by the 

 diameter of the acoustic transducer element. 



Transducer size— The frequency of choice can place one limit on 

 overall size because the resonant frequencies of the normally used PZT 

 cylinders vary linearly with diameter as shown in Table 8. Cylinder length 

 has relatively little effect; most elements used in USTs have length-to- 

 diameter ratios in the range of about 0.5 to 2.0. 



If the PZT cylinder is bonded rigidly to the inside of the transmitter 

 housing, the resonant frequency is lowered somewhat. For the air-filled 

 Mark V shark transmitter, a 40-kHz resonance is achieved with a 22.2-mm 

 (0.875 in.) OD element epoxied to the inside of the rigid-PVC transmitter 

 housing. 



Batteries— Usually the major factor limiting transmitter size reduc- 

 tion is the battery capacity required for the desired range and life. The most 

 relevant characteristic is energy density/unit weight, but other factors are 

 usually considered, such as cost, shelf life, initial voltage per cell, voltage 



Table 8. In-water resonant frequen- 

 cies (circumferential mode) of air- 

 backed, thin-walled cylinders of 

 PZT-4 (lead zirconate-titanate). 



