EQUIPMENT AT MOUNTAIN LAKES 



113 



more exact control of some of the electric and acous- 

 tic parameters. These are treated in detail in the fol- 

 lowing discussion. 



Adjustment and Maintenance of Electric System. 

 After the electric system is in thermal equilibrium, 

 the frequency scale on the oscillator must be set. As 

 mentioned previously, this is done at only two points, 

 76 kc and 2 mc, and the scale is then assumed to be 

 correct over the rest of the range. 



The output level is determined by connecting the 

 oscillator (or power amplifier, if used) to the trans- 

 mission measuring sel and finding the power dis- 

 sipated in 72 ohms. The set is calibrated to read a 

 specific power level of 133 db vs 10 -16 watt, but the 

 introduction of standardized attenuators between 

 the source and the set allows the measurement of 

 higher powers. The power level adjustment for the 

 oscillator may be made at any frequency, since its 

 output is Hat within 0.15 db oxer the entire range. In 

 the case of the power amplifier, which does not have 

 a flat response, the adjustment is usually made at 50 

 kc. 



The carrier frequency and modulation balance of 

 the detector are then adjusted. The sensitivity of the 

 receiving system (detector plus recorder) is adjusted 

 at a specific frequency, usually 50 kc, since the detec- 

 tor does not have a flat response. This adjustment is 

 made with the fine gain control so that, with the gain 

 dials of the detector set at zero, the recorder reads 

 directly the input level to the detector. The adjust- 

 ment results in a full-scale deflection of the recorder 

 for an input level to the detector of 135 db vs 10 -16 

 watt. The level of any recorded signal is, then, the 

 recorded level minus the gain of the detector as in- 

 dicated by the dial settings. 



With these adjustments, the equipment is ready 

 for use but checks of this nature must be made several 

 times each day to correct for minor changes which 

 may have occurred. 



The fact that the power amplifier and the detector 

 do not have flat frequency characteristics entails no 

 serious difficulty as long as the variations are rec- 

 ognized and corrections made. In most acoustical 

 measurements, comparisons are made between an 

 unknown and a standard. If the same equipment is 

 used with each instrument, the ratio between the two 

 response records is correct, although the actual read- 

 ings are not. Only the measurement and interpreta- 

 tion of specific quantities, such as current to a 

 projector, recpiire consideration of the frequency. 



Calibration of Standards. The principle of reci- 

 procity (see Chapter 5) was used in calibrating the 

 high-frequency standards. The actual testing pro- 

 cedure has already been described, so that only the 

 precautions peculiar to this range will be treated 

 here. 



With the short waves and narrow beams involved, 

 the hydrophone must be accurately oriented with 

 respect to the projector and both must be rigidly 

 clamped, as a very small movement may introduce 

 serious errors. Care must also be taken to have the 

 projector beam completely cover ihe active acoustic 

 face of the hydrophone. It is unfortunate that, in 

 many high-frequency units, the beam tends to 

 wander from the established acoustic axis as the fre- 

 quency is varied. This will introduce serious errors 

 unless the hydrophone is located far enough from the 

 projector to have only negligible variations in the 

 portion of the field being measured. This variation 

 in beam pattern may be detected by reorienting the 

 projector at various frequencies and noting the 

 change in direction of the axis. Effects such as these 

 necessitate the complex positioning equipment. 



The orientation of the projector and hydrophone 

 is made at the highest possible frequency because 

 of the increased accuracy afforded by the sharper 

 beam pattern. However, the possibility of beam 

 wandering must always be taken into consideration. 



The units should be far enough apart so that cor- 

 rections for spherical waves are unnecessary. (See 

 Chapter 5.) The method of testing for this effect, and 

 also for that of incomplete coverage of the hydro- 

 phone face by the sound field, is to take response runs 

 at several distances. The shortest distance at which 

 the inverse distance law holds determines the mini- 

 mum testing distance that should be used. 



To date, no instrument tested in the tank has 

 necessitated spherical wave corrections. Such an in- 

 strument could be calibrated in the pier test areas 

 which allow a much larger testing distance. 



If standing waves are present between transducer 

 faces, they can, in some instances, be eliminated by 

 rotating the face of the hydrophone through a small 

 angle. 



In measuring the current to the projector at these 

 frequencies, special precautions have to be taken be- 

 cause of the stray capacities involved. The circuit is 

 shown in Figure 47 and it is to be noted that both 

 leads are isolated from ground. 



Even though the ground between resistor and 



