PULSED TRANSDUCER 

 SIZE 250" DIA. 



THICKNESS .013" 

 MATERIAL LEAD 



ZIRCONATE 



RECEIVING TRANSDUCER 

 SIZE .250" DIA. 

 THICKNESS .026" 

 MATERIAL LEAD 



METANIOBATE 



IMPULSE 



OUTPUT 



Fig. 3. Acoustic current with carrier null. 



Fig. h. Acoustic behavior with wedge 

 backed transducer. 



Table I. Comparison of three acoustic systems 



Lead Zirconate 



Acoustic System 

 Pulse shape 



Rise time 



First peak voltage 



Voltage rate 



Conventional 

 Unsatisfactory 



100 microseconds 



6 mv 



Lead Metaniobate 

 good 



100 microseconds 



10 mv 



),000 v/seconds 100,000 v/seconds 



Lead Zirconate ("Wedge ) 

 Lead Metaniobate 



good 



90 microseconds 



8 mv 



90,000 v/seconds 



APPROACH TO DESIGN IMPROVEMENTS 



Various methods for improving the transducer 

 system were considered. The results sought were 

 that the pulse should have (l) a fast rise time 

 and (2) a first rise which exceeds all successive 

 transients . It appeared that the transmitting 

 transducer should be "damped" as much as possible 

 from the electrical side. For this purpose it 

 was felt that lead zirconate having a high 

 electro-mechanical coupling coefficient would 

 perform well. In addition some method of 

 destroying the resonance of the transducers, such 

 as by cementing a ceramic wedge to the back of 

 the crystal, seemed probable of success. On the 

 receiving end it appeared that a material with a 

 low electrical Q should be used and in addition 

 it might be useful to design the receiver crystal 

 to be anti-resonant at the resonant frequency of 

 the transmitting crystal to overcome the buildup 

 of carrier frequency energy. 



Fig. 3 shows the behavior of such an acoustic 

 circuit in which a transmitting crystal of high 

 coupling coefficient is driven from a very low 

 impedance source. The receiver is made of a low 

 Q material and is twice as thick as the trans- 

 mitting crystal in order to be ant i -re sonant. 

 Specifically, the transmitting crystal is of 

 lead zirconate titanate resonant at about 8 Mcps 

 while the receiving crystal is of lead metanio- 

 bate resonant at about 1+ Mcps . The pulse has a 

 rise time of approximately 100 nanoseconds, a 

 voltage rise rate of 100,000 volts per second 

 and possesses the desired characteristic, viz., 



that no portion of the pulse exceed the first 



rise. The amplitude of the first rise is 



10 millivolts and is approximately equal to the 



second half-cycle in the standard system shown 



previously. 



The carrier energy in this pulse is of no 

 value in the operation of the velocimeter and 

 some effort was made to further reduce it. For 

 this purpose a lead zirconate transducer was 

 cemented to a wedge shaped backing block and sub- 

 stituted for the transmitting transducer. This 

 configuration gave a slightly faster rise time 

 with slightly lower output but very much less 

 "carrier" energy as shown in Fig. h. Here the 

 rise time is about 90 nanoseconds with a voltage 

 rise rate of 9°) 000 volts per second. For direct 

 comparison the characteristics of the three 

 acoustic systems discussed are shown in Table I. 



All of the results shown here have been 

 observed with a direct water path from the trans- 

 mitting to the receiving transducer. As pre- 

 viously noted, reflections within the acoustic 

 path result in an interfering signal which 

 arrives at about the same time as the desired 

 signal. For example, assume we use a yardstick 

 as a time base and consider the acoustic delay 

 to be 11 inches and the electronic delay to be 

 1 inch. It can easily be seen that a signal 

 reflected by the receiver reaches the transmitter 

 after "22 inches" and reflects from it to arrive 

 at the receiver (for the second time) at '33 

 inches." The third succeeding pulse after passing 

 through the system only once arrives at the 



159 



