94 



USRL TEST STATIONS 



transducers functioning primarily as hydrophones, 

 and to calibrate transducers designed to function pri- 

 marily as projectors. Reversible transducers are cali- 

 brated functioning in each capacity. 



Reference data from the first procedure are re- 

 quired only when the test transducer is to be cali- 

 brated as a hydrophone. These runs generally pre- 

 cede observations on the test instruments and are re- 

 peated, in part at least, once each day during the test 

 period. 



Reference Data. When a device is to be calibrated 

 as a hydrophone, one or more standard projectors are 

 selected to cover the frequency range of the test in- 

 strument and the sound field established by each of 

 them is calibrated by at least two standard hydro- 

 phones. One of the hydrophones should have direc- 

 tional characteristics which will discriminate against 

 surface and bottom reflections, though reflective ef- 

 fects may be decreased by operating at short distances 

 within the limits discussed in Chapter 5. Whenever 

 certain frequencies are of particular interest, as in the 

 case of sharply resonant devices, it is advisable to 

 choose standard hydrophones having minimum ir- 

 regularities of response at these frequencies. 



Before reference runs are made, the angular posi- 

 tion of each projector is set where the maximum 

 acoustic output appears on the recorder. This train- 

 ing is done at a frequency where the beam pattern of 

 the projector is sharp, and at a distance sufficient to 

 minimize the effect of standing wave patterns be- 

 tween transducers. With low-frequency projectors, 

 the beams are usually broad enough so that careful 

 mechanical alignment in rigging is adequate. Once 

 adjusted, each projector remains undisturbed 

 throughout the tests. 



Runs are taken at several distances and agreement 

 between the computed and observed distance losses 

 indicates the absence of significant standing waves or 

 reflections. The sound field of each projector is then 

 computed at several frequencies, from the data ob- 

 tained with each standard hydrophone. With random 

 deviation from the mean, agreement within 1 db is 

 reasonable assurance of projector and hydrophone 

 stability. When the differences between the computed 

 sound pressures exceed 1 db, it is advisable to include 

 the data from a third standard in order to identify the 

 cause of the discrepancy. 



Receiving Response Measurements. Instruments 

 which have been designed for hydrophone operation 

 are calibrated in a sound field established by the pro- 



cedure described above. The instrument is oriented 

 acoustically before any test data are recorded, but the 

 device may or may not be tuned according to the 

 specifications of the program. 



On the basis of an exploratory observation, the 

 gain of the receiving amplifier is adjusted so that the 

 curve will remain on the chart as the frequency range 

 is covered. If the level range of the instrument ex- 

 ceeds that of the recorder, it may be necessary to 

 change the gain during the course of the run or to 

 repeat a portion of the curve at a different gain set- 

 ting. Runs at several test distances usually are re- 

 corded on the same chart and, if possible, with the 

 same gain. 



In the case of sharply resonant devices, supplemen- 

 tary records are made of the level at peak frequency 

 by slow manual variation of the oscillator. Measure- 

 ments at frequencies on each side are made until the 

 levels are 3 and 10 db below the peak response. These 

 observations permit determining the Q of the in- 

 strument. 



In general, the response of essentially nondirec- 

 tional apparatus is taken at several angular positions 

 such as 0, 90, 180, and 270 degrees. Comparison of 

 these runs may show inaccuracies in rigging, particu- 

 larly at low frequencies where diffraction effects are 

 not likely to occur. 



Hydrophone Coupling Measurements. The re- 

 corded data are a measure of the signal level at the 

 135-ohm input to the receiving system in db vs 10 — 16 

 watt. Since the instrument calibration is usually 

 wanted in terms of open-circuit voltage, or voltage 

 across a specified impedance, it is necessary to deter- 

 mine the relationship between these quantities and 

 the recorder level. This is obtained by injecting a 

 signal from a low impedance in series with the hydro- 

 phone circuit, thus simulating the voltage generated 

 under acoustic excitation. The difference in level be- 

 tween the injected signal and the input to the receiv- 

 ing amplifier determines the hydrophone coupling. 

 Typical circuit arrangements for observation of coup- 

 ling characteristics are shown in circuits A and B on 

 Figure 32. 



The hydrophone calibration may be required in 

 terms of the voltage delivered to a specific load re- 

 sistor. In this case, the hydrophone, bridged by the 

 required resistance, is connected to the coupling am- 

 plifier. The conversion of the recorded level to the 

 voltage at the input of the coupling amplifier re- 

 quires a knowledge of the relation of gain to fre- 



