EQUIPMENT AT MOUNTAIN LAKES 



89 



was comparison of instruments in a free field. Refer- 

 ence has been made in Chapter 5 to the advantages of 

 this method, and it seems probable that it will con- 

 tinue to be an important one. The following discus- 

 sion of testing procedures pertains to transducers in 

 general, the distinction between hydrophones and 

 projectors being made only when the nature of the 

 device affects the techniques. 



Required Background Information. In planning 

 an efficient program, it has been found expedient to 

 know in advance the size, shape, and other character- 

 istics of each device. The most essential information 

 is its actual performance. However, knowledge of the 

 end application and of the operating principle per- 

 mits emphasis to be placed on the characteristics of 

 primary importance. The following illustrates the 

 type of information to be included: 



1. Physical Characteristics 



Size of diaphragm 



Location of diaphragm center 



Location of center of rotation 



Configuration and type of active elements 



Position in use with and without auxiliary 

 gear 



Drawings showing dimensions and mount- 

 ing details 



Temperature and pressure limitations 



2. Electrical Characteristics 



Terminal impedances 

 Direct-current power requirements 



Working and maximum power input for con- 

 tinuous-wave and for pulse operation 



Tuning and associated network requirements 



Circuit schematics 



Frequency range 



Ordinarily there are points of particular interest 

 in each program. For example, it may be desired that 

 special attention be given to a restricted portion of 

 the frequency range, directivity patterns be taken 

 about certain axes or at specific frequencies, a de- 

 tailed investigation be made of the secondary or ter- 

 tiary resonances of a sharply resonant device, or the 

 performance of a projector be studied at specific 

 power inputs. 



Preparation for the tests may be expedited if all 

 this information is furnished well in advance of the 

 testing date. A person thoroughly familiar with the 

 instrument under observation can be of material as- 



sistance in the testing program, and one should be 

 present if possible. 



Rigging. Throughout the rigging, every precau- 

 tion is taken to protect the instrument from mechani- 

 cal shock or other injury, and crystal devices that are 

 injured by high temperatures should be shielded 

 from the sun as much as possible. In many cases a 

 large portion of the time is devoted to rigging the 

 test devices and associated gear. Each device presents 

 different rigging problems arising from disparities in 

 weight, size, frequency range, and the type of tests 

 planned. There are, however, many rigging consid- 

 erations common to most testing programs, and these 

 will be considered in the order in which they arise. 



The types of suspension for positioning transducers 

 are described in Section 6.2.1. The selection of a par- 

 ticular one is dependent on the physical character- 

 istics of the instrument as well as on the nature of the 

 observations to be made. In rigging for tests in which 

 the response is wanted only at a few angular positions, 

 a heavy instrument is mounted on 1-inch pipe, while 

 a light one usually is mounted on a hydrophone rod. 

 Since the rod is supported by gimbals and an anti- 

 shock mounting, it requires symmetrical loading. In 

 cases where the instrument construction does not 

 allow this loading, the 1-inch pipe may be used or 

 the device may be mounted on the rotator. When the 

 pipe is used, it is clamped in position by blocks on 

 rubber mountings. This protects against shock but 

 does not provide a completely free suspension. In any 

 case, care must be taken to have the whole assembly 

 hang in a truly vertical line, particularly if the dis- 

 tance between instruments is measured after they are 

 submerged. The test distance is taken to be that be- 

 tween the upper parts of the suspension rods, with 

 corrections for the position of the transducers rela- 

 tive to these rods. Where the testing distance is short, 

 small deviations from the vertical may result in ap- 

 preciable errors. Such irregularities in hanging may 

 also displace the instrument with respect to the acous- 

 tic axis of the standard instrument, introducing seri- 

 ous errors at frequencies where sharp beam patterns 

 occur. 



In rigging a test device to a pipe suspension, an at- 

 tempt is made to have the center of gravity of the 

 instrument lie on the major axis of the support and 

 the plane of its active face parallel to the axis. Adjust- 

 ment screws and small levels in the mounting blocks 

 facilitate the process, and counterweights may also be 

 employed. If the required adjustment is not too large, 



