supposed that the cylinder is virtually clamped to the metal 

 end pieces. All motion therefore takes place in the barium 

 titanate cylinder, and this ceramic material has a much 

 smaller mechanical loss than that associated with motion 

 in the rubber gaskets. This improvement is achieved at 

 the cost of a lower sensitivity of the transducer, but experi- 

 ments indicated that this loss was offset by the fact that the 

 transducers could be coupled closer without affecting the 

 cavity performance. 



The Neoprene -rubber solution used for dipping the 

 transducers is made so thin that bubbles are easily seen 

 and can be eliminated before the rubber sets. Also, it is 

 possible to prevent bubbles from forming if care is exer- 

 cised during the dipping process. Four or five coats are 

 apparently sufficient, since the transducers have been in 

 use for almost a year without any significant deterioration. 

 The loss introduced by the transducers can be neglected 

 for all conditions except where the total losses are an 

 absolute minimum. 



The block diagrams in figure 4A and 4B show two 

 possible measuring arrangements that were investigated. 

 Figure 4A is a "sing-around" circuit, where a signal pres- 

 ent in the cavity will be picked up by the receiving hydro- 

 phone, amplified, and reintroduced in the cavity. The 

 sing-around frequency will be a natural resonance frequency 

 of the cavity, if the electronic circuitry is properly adjusted. 

 The transmission loss from the input terminals of the trans- 

 mitting hydrophone to the output terminals of the receiving 

 hydrophone is approximately 80 db. The amplifier must 

 therefore supply at least 80-db amplification to establish 

 a sing-around condition, and experience has demonstrated 

 that it is extremely difficult to keep the system functioning 

 properly under field conditions. The equipment would fre- 

 quently sing-around on some spurious resonance frequency, 

 and very critical adjustments of the tuned circuit and trans- 

 ducer positions would be required before the measurement 

 could be obtained. 



Figure 4B shows a much simpler solution. The gen- 

 erator drives the transmitting cylinder, and the received 



