osc 



DRIVER 



VARIABLE 



REGULATED 



POWER 



SUPPLY 



/ I transducer! \ 



AF 



AMP. 



MIXER 



IF 

 AMP. 



DIODE 

 DET. 



LOCAL 

 OSC 



AUDIO 

 AMP. 



TRANSMITTER 



RECEIVER MAGMETER 



TAPE 

 RECORDER 



Fig. 1. System "block diagram. 



INSTRUMENT DESIGN 



VOLUME 



OF 



REVERBRATION 



RECEIVER 



TRANSMITTER 



Fig. 2. Transducer positioning. 



The transmitter section consists of a crystal 

 controlled oscillator driver whose output is 

 matched to a barium titanate transducer. The 

 receiving system consists of a broadband crystal 

 controlled superheterodyne receiver, transducer, 

 frequency meter (Model ^1-7991; Airpax Electronics, 

 Inc.) and tape recorder (Fig. l) . 



Both the transmitting and receiving transducers 

 were mounted on a head fixture which permitted 

 variation of the "interocular " distance between 

 the 2 units and experimental changes in the cross- 

 over point . The rear surface of the 1-inch 

 diameter transducer discs are air-loaded to elim- 

 inate back radiation. Theoretical beamwidth at 

 l/2 power points at the operating frequency is 

 given as 



O = 1.22 tj radians 



(2) 



where X = c/F and D is transmitter diameter. For 

 the test instrument X is 3-08 x 10" 2 and a is 

 O.836 . The beamwidth measured in a tank has been 

 found to be somewhat less than theoretical and 

 minor lobes are at least 60 db down in amplitude. 



A frequency of 5 Mcps was selected as a best 

 compromise between achieving narrow beamwidth, 

 ability to resolve small reflecting particles 

 and high Doppler sensitivity vs. increased attenu- 

 ation by the medium with high frequency. The 2 

 units of channellite 100 (barium titanate, Channel 

 Industries) were found to have resonance peaks 

 30 Kcps apart and more careful frequency matching 

 could undoubtedly improve the signal to noise 

 ratio and further reduce the transmitter power 

 requirement. A transducer spacing of 8 inches 

 with a crossover point of 10 inches (Fig. 2) was 



128 



