84 



USRL TEST STATIONS 



Figure 19. 30A transmission measuring set connected in 

 s\stem jack field. 



Transmission Measuring Set. The standard instru- 

 ment adopted for power level measurements is a 

 Western Electric 30A transmission measuring set. 

 The essential elements are a thermocouple, indicat- 

 ing meter, attenuators, and switching circuits. The 

 input impedance is 135 ohms and the set operates at 

 frequencies up to 150 kc. The readings may be varied 

 in increments of 1 db through a range of 90 db by 

 means of attenuator sections connected by dials and 

 switches. A high degree of convenience has been ob- 

 tained by the use of jack terminations for the individ- 

 ual circuit elements. The circuit of the instrument 

 provides for both gain and loss comparison paths, 

 selected by a switching key. 



The meter scale covers a range from — 10 to +3 db 

 on either side of a center zero. The design is such that 

 the zero reading means a power level of 130 db vs 

 10 _1G watt (1 milliwatt). An internal d-c circuit pro- 

 vides for the maintenance of the thermocouple and 

 metering circuit calibration. Recalibration of the 

 metering circuit may be necessitated by thermocou- 

 ple aging, temperature changes, and thermocouple 

 replacements. The overall accuracy of the transmis- 

 sion measuring set is ±0.1 db. 



Monitor Amplifiers and Speakers. Each system is 

 provided with means for listening to the received 

 signal. System 1 has an audio-amplifier with a maxi- 

 mum gain of about 60 db and an output power of 

 some 12 watts. This amplifier is used in conjunction 

 with a dynamic speaker. System 2 is equipped with 

 a monitor converter circuit that allows the use of 

 earphones at the output of the detector circuit. The 

 monitor output signal may also be sent through per- 

 manently installed lines to the power amplifier and 

 loud-speaker of the reproducer set. 



Reproducer Set. A transcriber is provided for re- 

 producing, electrically and acoustically, for calibra- 

 tion purposes, various types of water noises includ- 

 ing a number of ship noises. The transcriber has two 

 separate turntables, each with a reproducer for verti- 

 cal or lateral cut records at 33% or 78 rpm. A pream- 

 plifier associated with each head permits individual 

 control of level and of frequency weighting charac- 

 teristics. A 50-watt power amplifier is used for the 

 operation of a high-quality speaker. The frequency 

 response of the amplifier without equalization is sub- 

 stantially flat from 30 to 10,000 c. 



High-Pass and Loiv-Pass Filters. A number of highl- 

 and low-pass 600-ohm filters have been assembled 

 and connected through suitable switches so that 

 single units or combinations may be used. The high- 

 pass filters have cutoff frequencies of 0.2, 0.7, 2, 15, 

 33, and 60 kc; the low-pass filters, 0.7, 2, 5, 15, 35, 70, 

 and 150 kc. All filters have attenuation beyond the 

 cutoff frequencies of more than 50 db. 



Laboratory Intercommunication Systems. Each test 

 system has microphone-speaker communication be- 

 tween the laboratory and the piers. It has been ar- 

 ranged so that it is possible to contact each pier from 

 any one of the systems and vice versa. This intercom- 

 munication is required to permit proper coordina- 

 tion between the operators at separate locations 

 while making calibration measurements. 



Mechanical Features of Outdoor Facilities 



Handling Facilities. Overhead monorails are pro- 

 vided for trolleys to which chain hoists are attached. 

 Rail switches permit the placing of hoists carrying 

 rigged apparatus on storage spurs or in any arrange- 

 ment desired for testing. The hoists have capacities 

 up to 1 ton, and the rail height permits a lift above 

 the piers of 16 feet. 



Suspension Members. Standard laboratory hydro- 

 phones and small test devices are usually rigged on 



