Image from page 99 of "Basic methods for the calibration of sonar equipment" (1946)
Title: Basic methods for the calibration of sonar equipment
Publisher: Washington, D. C.
Contributing Library: MBLWHOI Library
Digitizing Sponsor: Boston Library Consortium Member Libraries
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84 USRL TEST STATIONS
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Figure 19. 30A tiansiiiission measiiiiiig set connected in system jack field. Transmission Measuring Set. The standard instru- ment adopted for power level measiirenicnts is a Western Electric 3()A 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 frecjuencies 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 con\enience 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-'" 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 o\erall accuracy of the transmis- sion measming 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. Systeirr 2 is equipped with a monitor converter circuit that allows the use of earphones at the oiuput 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. .\ transcriber is pro\ided for re- producing, electrically and acoustically, for calibra- tion pinposcs, \arious types of water noises includ- ing a inmtber of ship noises. The transcriber has two separate turntables, each with a reproducer for verti- cal or lateral cut records at 33i;, or 78 rpm. A pream- plifier associated with each head permits indi\ idual 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 frecjuency response of the amplifier without et|ualization is sub- stantially flat from 30 to 10,000 c. High-Pass and Loiu-Pass Filters. A number of high- and low-pass 600-ohm filters ha\e been assembled and connected through suitable switches so that single units or combinations may be used. The high- pass filters ha\c cutoff frecjuencies 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 be)ond the cutoff frequencies of more than 50 db. Laboratory Intercommunication Systems. Each test system has microphone-speaker communicatioir 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 \ersa. This intercom- munication is required to permit proper coordina- tion between the operators at separate locations while making calibration measurements. Mkchanical Features of Outdoor Facilities Handling Facilities. 0\erhcad monorails are pro- \ided 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 lalioratory hydro- phones and small test devices are usually rigged on
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