98 



USRL TEST STATIONS 



transducers not directly associated with preamplifiers. 

 The devices are usually suspended from the pier, well 

 below the lake surface, while the measurements are 

 made. Occasionally impedance measurements are 

 made with the transducer suspended in the acousti- 

 cally treated tank. Such data are checked for standing 

 wave effects by repeating a few observations from a 

 different position. The temperature of the water ad- 

 jacent to the transducers is always recorded and an 

 adequate soaking period is provided to permit com- 

 plete thermal equilibrium. 



Observations on wide-band transducers that have 

 no sharp resonances are taken to cover the frequency 

 range in increments which will permit the construc- 

 tion of accurate resistance and reactance curves. 

 Additional observations are taken on resonant trans- 

 ducers in the resonance regions, so that motional 

 impedance computations may be made. These ob- 

 servations include not only measurements at the 

 frequency of maximum response but also at frequen- 

 cies above and below this value until the response 

 is less by 0.5, 1,2, 3,5, and 10 db. 



After considering the bridges available for direct 

 impedance measurements, the usual procedure is to 

 take the series impedance, if its value is not over 

 10,000 ohms and one terminal can be grounded, and 

 the shunt admittance for larger values and devices 

 that cannot be grounded. Some attention must be 

 given to the choice of the frequency that should be 

 used. A description of the bridges is given in Section 

 6.2.8. 



Whenever possible, measurements are made at the 

 apparatus terminals but when additional cable must 

 be used, measurements are made of the device with 

 cable, and of the cable alone, so that a correction for 

 the latter may be applied. To facilitate making these 

 corrections, all bridge readings are expressed in the 

 same unit, that is, impedance or admittance. When 

 auxiliary apparatus is used, impedance measure- 

 ments are also made on it to allow the computation 

 of the values for the instrument itself regardless of 

 the circuit used. 



The impedance of a transducer may also be ob- 

 tained from the readings of the wattmeter and though 

 the accuracy is not as good as with the bridge, it af- 

 fords a continuous indication and permits the detec- 

 tion of all resonances. It has the further advantage 

 that impedance data is obtained at all power levels, 

 instead of at the low levels bridges require, and 



impedance as a function of power may be observed. 



Reciprocity Test Procedure 



Several times during the year a series of free field 

 reciprocity calibrations are made on all of the labora- 

 tory standard hydrophones. Comprehensive periodic 

 checks are essential, since most of the calibration 

 work is based on an accurate knowledge of the per- 

 formance of these instruments. A large part of the 

 preparation procedure is identical with that in com- 

 parison testing. The instruments are rigged, washed, 

 and oriented in the same manner. The required 

 measurements include response, hydrophone coup- 

 ling, and projector current, but the number and 

 sequence differ materially. A discussion of the the- 

 oretical considerations involved in reciprocity cali- 

 brations and of the general procedure used is 

 presented in Chapters 5 and 7. Actual procedures are 

 discussed herewith. 



Requisites. A reciprocity calibration requires the 

 use of a reversible transducer which obeys reciprocity. 

 Since two semi-independent calibrations can be ob- 

 tained with little extra effort by using a pair of such 

 transducers, this procedure is considered. It may be 

 necessary to use several pairs of transducers to cover 

 the frequency range from 15 c to 150 kc. In each test 

 run, one of the given pair operates as a hydrophone, 

 while the other furnishes the sound field. In a re- 

 ciprocity calibration it is necessary to know the open- 

 circuit voltage of the instrument, although it may be 

 more convenient to measure the voltage across a 

 known impedance and calculate the open-circuit 

 value. The driving and receiving impedances are 

 selected to provide the most uniform frequency re- 

 sponse. The equations given in Chapter 5 require a 

 knowledge of the distances between the acoustical 

 centers. The selection of a center is arbitrary but, 

 once selected, must remain the same throughout the 

 test. It is usually chosen to approximate the center 

 of the spherical waves which the transducer produces 

 at large distances and for this reason, runs are taken 

 at several hydrophone-projector separations. From 

 the distance-loss relationships, the effective acoustic 

 center of the transducer can be determined. The runs 

 also assist in evaluating the effect of reflections and 

 standing waves. 



Testi?ig Procedure. It is indicated in Chapter 5 that 

 there is no simple a priori test that determines wheth- 

 er or not a particular transducer obeys the reciprocity 



