EQUIPMENT AT MOUNTAIN LAKES 



97 



position switch which determines that the outputs 

 shall match, or chooses the one that shall be made 

 to lag. 



The patterns usually taken for the 151)1 charac teri- 

 zation of a transducer are for halves in phase with no 

 shift, halves in opposite phase with no shift, halves in 

 phase with one shifted to lag, and halves in phase 

 with the other shifted to lag. The gain chosen lor the 

 first condition should be maintained for the others. 

 The first two patterns are usually recorded on one 

 chart and the second two on another. 



Pattern investigations of hydrophones usually are 

 made with a continuous sinusoidal frequency, but 

 may be made by pulsing within the limits discussed in 

 Chapter 5. It is of interest occasionally to take pat- 

 terns representing listening conditions with a broad 

 noise band covering approximately the audio range. 

 A 6-kc band centered at 4 kc is most satisfactory. 



Transmitting Response Measurements. In obtain- 

 ing data on transmitters it is not necessary to establish 

 a definite sound field. The calibration is made with 

 standard hydrophones, selected on the basis of fre- 

 quency coverage and uniformity of response at speci- 

 fied frequencies. Most of the measurements are made 

 with only one standard, but this is checked by at least 

 one other. This is not only to check the standard hy- 

 drophone but also to test the projector stability. Most 

 of the preliminary observations are made at low 

 power levels and then extended to maximum values. 

 Projectors are usually calibrated with continuous 

 sinusoidal signals, but tests may be made with pidsed 

 ones, as described above. The impedance from which 

 the projector is driven is selected to agree with the 

 recommended value. A convenient available power 

 level is 160 db vs 10 -16 watt, but it must never exceed 

 the recommended maximum. (See Figure 29.) 



Projectors that require an external tuning circuit 

 for power factor improvement are calibrated both 

 with and without this circuit. Some projectors re- 

 quire a polarizing current, usually supplied through 

 a universal-type junction box. This box provides for 

 the adjustment and metering of the current, and also 

 incorporates adjustable capacitance networks made 

 flexible by a plug-in design. 



Preliminary observations include checks on leak- 

 age, cross talk, noise, overloading, angular orienta- 

 tion, and required circuit gain. Following these, re- 

 sponse data are taken, usually at two distances, and 

 coupling measurements are made for the standard 

 hydrophone used. 



/TV L 



TEST STATION 



PROJ NO . £><? — DATF ?■ *- *' 1 -VcOPIES TO STATION FILE AND N. Y OFFICL 



mii 1 1 -mi i h '. 



Output- vs /"pimi/Y J/ypi^r 



fo^ QG-3 Q^Dr^ero^ - f'vrrno 



(S £e 



t ( 17 <" ■ '•'(" i.-t J- ( V' 



Figure 35. Typical data sheet, showing projector output 

 versus electric power input. 



Transmitting Directivity Patterns. Directivity pat- 

 terns about one or more axes of rotation are required 

 in a projector calibration to determine the radiation 

 field. The procedure for taking these with the polar 

 recorder has already been given under Receiving 

 Directivity Patterns. 



Load Characteristics. Load-run observations to de- 

 termine the relationships between the various projec- 

 tor characteristics and the impressed power may be 

 made by means of the circuit arrangement shown in 

 Figure 34. These data should include observations of 

 linearity, frequency shift, distortion, and impedance. 

 .Additional patterns may be taken to observe the 

 variation of any quantity as a function of power. 



Figure 35 is a copy of an original data sheet show- 

 ing typical load-run observations. The time entries 

 are indicative of the soaking periods allowed for ade- 

 quate thermal stability to be reached following each 

 change in impressed power. A description of the watt- 

 meters with their circuits and theory is given later in 

 this chapter in Section 6.2.8. 



Impedance Data. Impedance data are taken on all 



