460 MECHANICAL AND ACOUSTICAL SENSES 



the PZT transducer, (2) the efficiency of the PZT transducer in converting 

 electrical power into acoustic power, and (3) the directivity index (DI) of 

 the PZT element in radiating sound outward (see section on Hydrophone 

 directivity). 



The following relationships obtain: 



1 electrical watt = 1 acoustical watt (if energy conversion is 100% 

 efficient). 



1 acoustical watt radiated omnidirectionally (spherical spreading) = a 

 signal strength (SPL) of +170.8 dB re 1 jiiPa at 1 m distance (absorption 

 can be neglected at this short distance). 



For example, the CSULB Mark V shark transmitter draws approximately 

 500 mA at 8 V (= 4 electrical watts) from the battery during the power 

 pulse. If amplifier efficiency and transducer efficiency are each assumed to 

 be 50%, then 4 W from battery = 2 W into PZT = 1 acoustical watt into the 

 water. For the broadly toroidal radiation pattern produced by the 

 thin-walled PZT cylinder, the directivity index can be considered about 1.5 

 dB (for full omnidirectionality, DI = dB). 



The following equation may then be used: 



SPL at 1 m = 170.8 + 10 log acoustic power + DI 



= 170.8 + 10 (log 1.0) + 1.5 m 



= 170.8 + + 1.5 { ' 



= 172.3 dB re 1 /iPa (in the radial direction) 



This calculated value of 172.3 dB re 1 jiiPa is close to the measured value 5 

 of 171.8 dB re 1 jiiPa, indicating that the efficiency estimations of 50% were 

 about correct. 



Pulse length— It is the general experience of tracking personnel that 

 increased pulse length (pulse width) results in increased signal -recognition 

 range, at least up to lengths of roughly 20 ms. Reasons for this are complex 

 and varied but partly involve the greater amount of energy in the longer 

 pulses and the lesser chance that all or most of this energy will be lost due to 

 transmission anomalies such as time stretching and fluctuating, multipath- 

 induced interferences. Another reason is that short pulses sound more like 

 clicks and are thus more difficult to distinguish from other short 

 ambient-noise clicks, such as those produced by snapping shrimp. As pulse 

 length Increases to more than about 5 ms, the listener becomes increasingly 

 aware of the tonal (frequency) quality of the pulse, and this aids 

 considerably in recognition. Still another reason involves the limitations set 

 by the slow response time of narrowband receivers (see section on Receiver 

 Characteristics). 



4 dB/ibar = decibels re 1 dyne/cm 2 = dB re 1 /xPa - 100. 



°Sound level measurement courtesy of International Transducer Corp., Goleta, Calif. 



