118 Lecture 6 
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O(I m) 5 (1.78 m) 10 (3.16 m) 15(5.62 m) 20 (10 m) 
Distance en db: re: |m 
ctest & dire 20 log (rmatres) 
Fig. 6.16. Acoustic field between two transducers: Panoramic Projector OUM-15 and P30P-209, 
and 14 m. Between 0 and 20 mthe measurements have to be made point by point. 
Thus, heavy and large-sized transducers canbe calibrated free from the Fresnel 
zone. Figure 6.16 shows the accoustic field between the omnidirectional trans - 
ducer OUM 15 and P30P at 15 kcps. 
Two concentric shafts are available for the testing of sonar domes. The 
central shaft carrying the transducer is rotated by remote control in the same 
way as described above. When studying vertical directivity patterns, depth and 
tilt of the test transducer can be adjusted by a rack gearing. 
The standard transducers are checked by reciprocity calibration every three 
months. They are used in the frequency range from 6 to 34 kcps. 
The instrumentation is shown in Fig. 6.17. The Test Stand provides facilities 
for the following measurements: (1) reciprocity calibration, (2) calibration by 
comparison, (3) field measurements, (4) measurement and recording of direc- 
tivity patterns, and (5) study and tests of sonar domes. This stand consists of: 
(1) A Muirhead generator, (2) a 20-w power amplifier, (3) a milliammeter with 
thermocouple for reciprocity calibration, (4) three Philips voltmeter-amplifiers, 
(5) a calibration amplifier with an amplifier and a series of octave-band filters 
covering a range from 3 to 70 kcps. This amplifier can be replaced by another 
with a bandwidth of 450 cps containing tuning circuits and covering a range from 
3 to 50 kcps. 
The Impedancemeter is used for measuring in the range from 3 to 50 kcps: 
(1) impedances (R,) up to 500 kohms and (2) capacitances (C,) up to lyf. 
