EQUIPMENT AT MOUNTAIN LAKES 



75 



Figure 9. Interior of transmitting booth. 



Figure 10. Interior of receiving booth. 



frequency characteristic is essentially flat through 

 the range from 15 c to 150 kc and the voltage gains 

 for the unbalanced and balanced input conditions 

 are approximately +0.5 db and — 6.0 db, respectively. 



A battery supply and coupling circuit is provided 

 for the frequently used standard hydrophones such 

 as the 3A, 5C, and 5D types. A metering panel per- 

 mits monitoring of all A and B voltages and currents. 

 Switches and jack-terminals provide for measure- 

 ments of various quantities, such as response, coup- 

 ling, and available power. To calibrate a hydrophone 

 on open circuit requires a knowledge of the loss in 

 the coupling circuit. The procedure for determining 

 this is to place in series with the hydrophone a re- 

 sistance which is very small in comparison with the 

 resistance of the instrument. A variable oscillator of 

 low voltage is applied to this resistor and the signal 

 is carried through to the recorder as though from the 

 hydrophone itself. After the range of frequencies has 

 been covered, the same voltage is connected directly 

 to the recorder and the range swept over again. The 

 difference between the records in db is the loss in the 

 coupling circuit. 



Various types of supplementary apparatus are fre- 

 quently required. One such device is a portable bat- 



tery-operated preamplifier that may be placed at the 

 edge of the testing area in order to reduce the length 

 of the hydrophone cable. Another is an underwater 

 preamplifier, operated from the battery for use with 

 high-impedance instruments, such as tourmaline 

 gauges (tourmaline crystal hydrophones). This am- 

 plifier, mounted in a watertight housing, is equipped 

 with cable glands lor hydrophone leads, battery sup- 

 ply connections, and lines for calibration and output 

 signals. Several special battery supply, coupling, and 

 metering circuits for miscellaneous standard hydro- 

 phones are available. Portable low-power d-c supply 

 circuits, suitable for various preamplifiers, have been 

 designed and are discussed later. 



Receiving Amplifiers 



In both system 1 and system 2, high-gain, wide- 

 band, low-noise-level amplifiers are used to increase 

 the incoming signal to levels suitable for recording. 

 The frequency characteristics of these amplifiers are 

 Hat within 0.2 db from 15 c to 150 kc. 



The coaxial lines are coupled to these amplifiers 

 by magnetically and electrically shielded input trans- 

 formers, with the input winding balanced to ground. 

 These receiving amplifiers use four low-noise-level 



