EQUIPMENT AT MOUNTAIN LAKES 



99 



principle. The most satisfactory substitute is a cross 

 check between two reversible instruments separated 

 by a fixed distance. With one functioning as a hy- 

 drophone and the other as a projector, the ratio is 

 obtained of the open-circuit voltage developed by 

 the one to the current supplied the other. The elec- 

 tric connections are then interchanged and the runs 

 repeated, giving the same ratio with the function of 

 each reversed. If the two transducers obey reciprocity, 

 these ratios will be ecpial at each frequency. It is pos- 

 sible that these ratios may be equal, even though the 

 transducers do not obey reciprocity, but there is little 

 chance of this unless the transducers are identical, in 

 which case they may violate the reciprocity principle 

 to the same degree. 



When reciprocity has been established, the trans- 

 ducers are placed at opposite ends of the testing area 

 with no change in their orientation or depth. The 

 hydrophone to be calibrated is then mounted on a 

 movable carriage between the two and tested against 

 each in turn. The procedure outlined in Chapter 5 

 is then carried through, using each transducer as a 

 projector and as a reversible transducer as described. 

 This procedure yields two semi-independent recip- 

 rocity calibrations of the hydrophone. It is usual to 

 test successively all hydrophones of the same type, 

 since this procedure entails a minimum number of 

 changes in rigging and electric connections. 



Other Free Field Acoustical Observations 



In addition to the usual calibrations, acoustic stud- 

 ies are made which involve somewhat different test- 

 ing techniques, such as observations on domes, baffles, 

 generators of complex acoustic waves, and complete 

 echo-ranging systems. 



Dome Studies. When a dome is submitted for in- 

 vestigation, the projector to be used with it may not 

 be included and a suitable one must be selected. It 

 may be a wide-band device or a sharply resonant 

 one, but it must have low side lobes at the frequencies 

 of interest and have as nearly as possible the same 

 size and directivity as the projector for which the 

 dome was designed. This is very important, since a 

 projector with unsuitable directional characteristics 

 can completely obscure important dome charac- 

 teristics. 



The transducer is usually tested first without the 

 dome by rigging to the inner shaft of the rotator. The 

 measurements ordinarily include frequency response 



as a hydrophone or as a projector and directivity 

 patterns as a hydrophone. The character of the trans- 

 ducer will largely determine the frequencies at which 

 the dome is tested. If the transducer is sharply reso- 

 nant, patterns may be taken only at the resonant fre- 

 quencies. 



Following these tests the dome is thoroughly 

 cleaned and rigged to the outer shaft of the rotator 

 with care being taken to insure correct positioning of 

 dome to transducer. The assembly is then debubbled 

 and allowed to reach temperature equilibrium. 



The type and character of the test data required 

 for a comprehensive investigation are illustrated in 

 Table 1. Figure 36 shows the various configurations 

 of the acoustic gear. Because of reciprocity, as shown 

 in Chapter 5, the effect of the dome, the baffle, and 

 their surroundings on the response and directivity 

 of the transducer is the same on sending and receiv- 

 ing. For practical reasons, receiving is preferred. 



There are considerations in making dome directiv- 

 ity tests which require somewhat different arrange- 

 ments. To investigate the effects of either a given 

 target or a noise source located at a definite bearing, 

 the nose of the dome is set at the bearing and the 

 transducer alone is rotated as indicated in Figure 

 36D. The same procedure with the dome reversed, as 

 shown in C, is used to simulate the propeller noise 

 from one's own ship. The effect of the dome on the 

 pickup of water noise and reverberation for any par- 

 ticularly trained position <£ of the transducer can 

 best be shown by rotating the dome and projector 

 together with the angle <j> between them fixed during 

 each trial (Figure 36E). This plan calibrates the re- 

 sponse of this combination of dome and transducer 

 to noise from any direction. 



Accurate positioning of the dome and hydrophone 

 is easily accomplished by means of the rotator and 

 turntable. Either rotator shaft may be locked in posi- 

 tion while the other is rotated. It is thus possible to 

 secure the required angular separation between the 

 hydrophone face and the dome nose. By engaging 

 both shafts, the whole assembly may be rotated while 

 the relative angular position of dome and hydro- 

 phone remains fixed. 



Baffle Studies. The reflective and absorptive prop- 

 erties of baffles may be studied independent of domes. 

 While no direct measure of absorption is made, this 

 characteristic may be estimated from reflection and 

 transmission measurements. The testing arrange- 



