EQUIPMENT AND FIELD FKOCEDURES 



273 



QCH-3 

 PROJECTOR 



FILTER 



JUNCTION BOX 



NO. I 



^ 



SELF-EXCITED 

 DRIVER 



SIGNAL 

 KEYING BOX 



BEAT FREQUENCY |_ 

 AMPLIFIER 



LOUD 

 SPEAKER 



MECHANICAL 

 KEYING CONTROL 



ATTENUATION 

 KEYING BOX 



II I 

 -Li 



TIMING TRACE 

 CONTROL CIRCUIT 



400 CYCLE 

 FORK 



PRE-AMPLIFIER 



4 



VOLTAGE t= 

 AMPLIFIER 



CATHODE-RAY 

 OSCILLOSCOPE 



CAMERA 



Figure 1. Schematic arraneement of apparatus employing QCH-3 units (eauipment A of text) 



when projecting sound and its better response when 

 receiving. Most of the more recent reverberation 

 studies off San Diego have been performed with 

 crystal transducers. 



The transducer alone is not capable of sending out 

 pulses and detecting incoming sounds. There must 

 also be equipment which delivers electrical energy to 

 the projector and amplifies and modifies the small 

 electrical impulses at the terminals of the receiver, 

 thereby converting them into a detectable form. In 

 the projector circuit of this auxiliary electric equip- 

 ment there is an oscillator which generates an elec- 

 trical signal of the desired frequency, and a power 

 amplifier. In the receiver circuit, there is usually a 

 preamplifier which takes the output at the terminals 

 of the hydrophone and amplifies it somewhat, and 

 then another amplifier, whose output is connected to 

 the recording mechanism. Somewhat different re- 

 cording techniques have been used by UCDWR and 

 WHOI. At UCDWR, the voltage developed by the 

 returning reverberation is usually fed into a cathode- 

 ray oscillograph so that the instantaneous deflection 

 on the cathode-ray screen is proportional to the 

 instantaneous voltage developed in the receiver. The 

 cathode-ray deflection as a function of time is re- 

 corded in permanent form by the use of a camera 

 with continuously moving film. In the technique 

 used until very recently at WHOI, the current gener- 

 ated in the gear by the reverberation activated a 

 galvanometer, which in turn threw a light beam on 



a moving roll of sensitized paper. The newest WHOI 

 equipment uses a cathode-ray oscillograph and a 

 camera, but is different from UCDWR equipment in 

 a number of other features. Usually inserted some- 

 where in the receiving circuit is heterodyning equip- 

 ment, which conv^erts the incoming high-frequency 

 energy into energy within the range of audible fre- 

 quencies and thus permits listening to the returning 

 reverberation by ear. This heterodyned signal may 

 be recorded, if desired. 



In the following paragraphs, we shall discuss in 

 more detail the principal electronic setups which 

 have been used in making reverberation measure- 

 ments. For convenience, these setups will be identified 

 bj"^ the letters A, B, C, D, E. Setups A and B were 

 used at UCDWR prior to January 1943; in later 

 UCDWR studies, setup C was used aboard the 

 Jasper and setup D abroad the Scripps. Setup E has 

 been used at WHOI. 



Equipment A 



This equipment ^ employed a pair of QCH-3 

 transducers, one used as a projector and the other as 

 a receiver. Driven at 23.45 kc, the QCH-3 projector 

 generated a sound pressure on the axis of 88.5 db 

 above 1 dyne per sq cm at 1 yd with a total acoustic 

 power output of 1.4 watts. A block diagram for this 

 system is given in Figure 1. The ping was started and 

 completed by closing and opening the ground circuit 

 in the oscillator-driver stage by use of an electronic 



