EQUIPMENT AT MOUNTAIN LAKES 



91 



LOG 



Figure 29. Sample log sheet. 



ing by ihe noise. Overloading of the preamplifier as- 

 sociated with the test hydrophone should lie investi- 

 gated, particularly in the case of devices having a high 

 sensitivity. Overloading may be detected with a 

 cathode-ray oscilloscope which shows a nonlinear re- 

 lation between the input pressure and the output 

 signal. The sound pressure on the hydrophone may 

 be reduced by decreasing the driving power of the 

 projector or by increasing the test distance. 



Cross-talk levels may be investigated by spacing the 

 projector and hydrophone about 3 meters and by ad- 

 justing the system gain controls to a high recorder 

 level at some convenient frequency. A rapid change 

 in frequency, effected by manual operation of the 

 dial, should show an abrupt drop in the output signal 

 because of the effective detuning action of the detec- 

 tor. The magnitude of this change is a direct measure 

 of the margin between the acoustic signal and the 

 electric interference. 



Test Observations. The recorder charts of runs for 

 acoustical and electrical data are supplemented by de- 

 tailed log and circuit sheets giving an index to the 

 series of tests, identifying the various runs, and noting 

 the instruments used and the circuit arrangements. 

 To facilitate the recording and analysis of the data, 

 each run is numbered according to the test sequence, 

 and each chart is given an identifying letter. Other 

 entries on the log sheets include the water tempera- 

 ture, testing depth, pier positions of the instruments, 

 and the time at which each run is taken. Sample log 

 and circuit sheets are shown in Figures 29, 30, and 3 1 . 

 In addition to the data, descriptions of the test de- 

 vices are included in the form of blueprints and 

 schematics furnished by the maker of the apparatus, 

 rough sketches made at the laboratory, or photo- 

 graphs. 



Test observations are usually made to calibrate a 

 sound field with a standard hydrophone, to calibrate 



