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1、ANSI S1.20-1988 (ASA 75-1 988) (Revision of ANSI S1.20-1972) REAFFIRMED BY ANSI ON: MAY 2 8 AMERICAN NATIONAL STANDARD PROCEDURES FOR CALIBRATION OF UNDERWATER ELECTmw ACOUSTIC TRANSDUCERS/CiWPipTeTd2) xexpU(o/c)(d, +d3-d2)Y2. (11) The SI derived unit of free-field current sensitivity (Mi H is the a
2、mpere per pascal ( M a ) . The free-field current sensitivity level is expressed in decibels, re: one ampere per micropascal ( 1 NpPa = l Npbar). See Sec. 4 of American National Standard S1.8-1969 (R1974). 5.3.2.4 Transmitting Current Response See 7.16 of American National Standard S 1.1- 1960 (R197
3、61.1 The magnitude of the transmitting current response of a projector P at frequency f and at the distance 1 m in a specified direction is given by The S I derived unit of transmitting current re- sponse (Si is the pascal meter per ampere (Pa.m/ A). The transmitting current response level is ex- pr
4、essed in decibels, re: one micropascal meter per am- pere ( 1 pPa-m/A). See Sec. 4 of American National Standard S 1.8- 1969 (R 1974). 3 i FIG. 2. Measurement framework for supporting the three transducers in-line. Copyright Acoustical Society of America Provided by IHS under license with ASA Licens
5、ee=IHS Employees/1111111001, User=Wing, Bernie Not for Resale, 04/18/2007 04:52:51 MDTNo reproduction or networking permitted without license from IHS -,-,- ANSI S1.20-1988 5.3.2.5 Transmitting Voltage Response See 7.15 of American National Standard S1.l-1960 (R1976). With the arrangements shown in
6、Fig. 1 and the vol- tage and current measurements of 5.3.2.3, the trans- mitting voltage response (Se ) , , of projector P at fre- quency f and at the distance 1 m in a specified direction is given by The SI derived unit of transmitting voltage response (Se is the pascal meter per volt (Pa.m/V). The
7、 transmitting voltage response level is expressed in decibels, re: one micropascal meter per volt (1 pPa*m/V). See Sec. 4 of American National Stan- dard S1.8-1969 (R1974). 5.3.2.6 Sources of Error in Reciprocity Calibrations There are several possible sources of error besides those that can be iden
8、tified by the tests for free field (5.3.1) and for a reciprocal transducer (5.3.2.1), and these must be examined. In a reciprocity calibration, three currents and three voltages must be measured, as seen in the foregoing sections. Generally, the current is measured by the voltage generated in the se
9、condary of a current transformer or it is measured as the voltage drop across a standard resistance placed in series with the transducer whose current is to be measured (5.3.2.6.1). Thus a voltmeter can be used for all elec- trical measurements. Because the equations for sensi- tivity and response t
10、hen show an equal number of vol- tages in the numerator and the denominator, the voltmeter need not be a standard. It must, however, be linear, stable, and should have an impedance that is very large with respect to that of all measured voltage sources. All electrical measurements must be made at th
11、e same terminals of the transducer. Because three experimental arrangements are required for a calibra- tion, stability of the measuring equipment is essential. 5.3.2.6.1 Current Measure. Two methods for measur- ing transducer currents are in general use. In the better method, the current is measure
12、d by the voltage genera- ted in the secondary of a current transformer when the primary is placed in series with the transducer and of- ten in the high side of the circuit. A shielded trans- former of toroidal configuration having a primary of low impedance and low capacitance to the secondary and t
13、o the ground is recommended. In the other meth- od, the current is measured as the voltage drop across a standard resistance in series with the transducer and 7 generally in the ground side of the circuit. This resistor must be located at the transducer terminal, its value must be less than 1% of th
14、e value of the resistive com- ponent of the complex impedance, and it must never be more than a few ohms in order to avoid noticeable errors resulting from additional currents through dis- tributed capacitance to ground. A third method has been used to measure the current in small, low-fre- quency,
15、coupler projectors. A large standard capacitor is placed in series with the projector, and the current is computed from the measured voltage across the ca- pacitor and its reactance. The method can be used to l Hz or less provided the capacitor is of sufficiently low dissipation. 5.3.2.6.2 Hydrophon
16、e Output Voltage. The impedance of the voltmeter should be greater than approximately loo0 times the impedance of the hydrophone for mea- surements involving phase and greater than approxi- mately 100 times the impedance of the hydrophone if only magnitude is desired; if it is not, a correction to o
17、pen-circuit voltage must be made (see 5.8 1. The mea- sured voltages may range from a few microvolts to volts. Acoustic and electrical interference should be eliminated, if possible, by modifying the measuring conditions. The undesired, interfering signals are those transmitted electromagnetically a
18、nd acoustically by in- direct paths. The pulse technique can be used to sepa- rate the desired, direct-path acoustical signal from the undesired, interfering signals. A gated receiving sys- tem measures the direct-path acoustical signal and blocks out the interference. The measured signal can be det
19、ected by means of an oscilloscope. In sweep-fre- quency, continuous-wave measurements, the interfer- ence that results from two signals from a common source (acoustic reflections or electromagnetic trans- mission and the desired signal) can be resolved by computation (see 5.1 1.2). 5.3.2.6.3 Efectiv
20、e Acoustic Center. American National Standard S1.1-1960 (R1976) defines the effective acoustic center of a projector as “the point from which spherically divergent sound waves, observable at re- mote points, appear to diverge.” In practice, the acoustic center is the point used for positioning the p
21、rojector and measuring distances, and through which any axis of rotation must pass. Except for spherical projectors, the acoustic center must be determined ex- perimentally. Some logical point is selected and then tested by a distance-loss experiment; that is, the sound pressure from the projector m
22、ust be inversely propor- tional to the distance from the acoustic center to the point of measurement, or the 6-dB-per-double-distance rule must apply, for all orientations. Copyright Acoustical Society of America Provided by IHS under license with ASA Licensee=IHS Employees/1111111001, User=Wing, Be
23、rnie Not for Resale, 04/18/2007 04:52:51 MDTNo reproduction or networking permitted without license from IHS -,-,- STD=ASA SI-20-ENGL L788 9 0181440 UOh42Al O74 D 8 AMERICAN NATIONAL STANDARD Most projectors are symmetrical in an up-down and right-left sense; therefore, errors in selecting the acous
24、tic center usually are in the axial direction (x axis in 5.7.2.1). Such position errors usually are only a few percent or less of the projector-to-hydrophone dis- tance, and, therefore, cause measurement errors of only a few tenths of a decibel. For this reason, the se- lection of the acoustic cente
25、r can be quite arbitrary, if long distances between transducers are feasible. The concept of an acoustic center applies by defini- tion only to a projector. A hydrophone is presumed to be in a sound field of plane progressive waves where the sound pressure amplitude is independent of posi- tion. In
26、practice, of course, this is not true, and the hy- drophone position is that of its acoustic center when viewed as a projector. The test of a hydrophones acoustic center is the same as that for a projector; that is, when the hydrophone is placed at two or more dis- tances from a source of spherical
27、waves, its output should be inversely proportional to the distance. 5.3.2.6.4 Transducer Alignment. In a reciprocity cali- bration with highly directional transducers, a slight misalignment may be a significant source of error. It is essential that the direction of the measured sensitivity or respon
28、se of a transducer relative to its principal axis be the same for each arrangement, whether the trans- ducer is used as a projector or as a hydrophone. If this alignment is accomplished acoustically, it must be per- formed each time at the same frequency, since the ori- entation of the maximum may b
29、e a function of fre- quency. 5.3.2.6.5 Source Proximity. Spherical wave divergence, as would be obtained from a true point source, is ap- proximated by real transducers only beyond a mini- mum source distance such that where a is the largest radius of a piston source or half the length of a line sou
30、rce, d is the source distance from the field point, and A is the wavelength of the sound. Beyond this minimum distance from a circular pis- ton or line source, the sound pressure at a point receiv- er will be within 4% of that calculated by assuming spherical divergence. This minimum distance also e
31、x- i s t s for a finite receiver and a point source. Physically, the requirement that the transducer look like a point source means that the variation in distance from the measuring position to any place on the surface of the transducer must be small in comparison with a wave- length or the distance
32、. The expressions are equally val- id for a transducer shaded to reduce minor lobes, be- cause the effective radius or half-length then is less than a (see Ref. 3). 5.3.3 Procedures for Secondary Sensitivity and Response Calibrations In a secondary calibration, the unknown sensitivity or response is
33、 determined by reference to a standard hydrophone. The method requires less time and equip- ment because only two electrical measurements are made. Careful selection of the measuring transducers for optimum directivity can compensate for lack of ideal free-field conditions. Accuracy and reliability
34、can be increased by averaging the results of measurements made with two or more standard hydrophones. 5.3.3.7 Free-Field Voltage Sensitivity The secondary calibration of a hydrophone requires a projector that need not be reversible nor linear, but linearity is desirable. The sensitivity of the stand
35、ard hydrophone should be stable and free from hysteretic effects resulting from the expected variations of am- bient temperature and pressure. If boundaries disturb the free-field conditions, then an error is introduced. The pulsed-sound technique can be used (see Appen- dix A) in the kilohertz and
36、megahertz frequency ranges. At lower frequencies, the error can be reduced by using a standard hydrophone whose angular devi- ation loss see 7.8 of American National standard Sl.1-1960 (R1976)I is equal to that of the measured hydrophone. A comparison is made by placing the standard and the unknown
37、hydrophones successively in the same position in the sound field of the projector. The magni- tude of the free-field voltage sensitivity ( M , of the measured hydrophone is given by l ( K ) H l = I(M,),fl* - Y 13 (15) where (M, )ref = free-field voltage sensitivity of the refer- ence or standard hyd
38、rophone, eH = open-circuit voltage output of the measured hydrophone, eref = open-circuit voltage output of the standard hydrophone. NOTE: If the standard and unknown hydrophones are so differ- ent i n size and directivity that the measurenents cannot be made with the two of them successively at the
39、 same position, a correction for distance loss must be applied to one of the open-circuit voltages. Copyright Acoustical Society of America Provided by IHS under license with ASA Licensee=IHS Employees/1111111001, User=Wing, Bernie Not for Resale, 04/18/2007 04:52:51 MDTNo reproduction or networking
40、 permitted without license from IHS -,-,- STD-ASA SII*O-ENGL 1988 018I1440 000111282 TOO I ANSI S1.20-1988 9 5.3.3.2 Free-Field Current Sensitivity The SI derived unit of transmitting current response (Si) is the pascal meter per ampere (Pa.m/A). Transmittingcurrent response The procedure is similar
41、 to that outlined in 5.3.3.1: level is in decibels, re: one microDu meter m m e (1 pPa.m/A). See Sec. 4 of American National Staidard s1.8- 1969 (R1974).1 where (Mi ) , = free-field current sensitivity of the measured hydrophone, (Me Iref = free-field voltage sensitivity of the standard hydrophone,
42、i, = short-circuit current output of the measured hydrophone, and erer = open-circuit voltage output of the standard hydrophone. See statement after Eq. ( 1 1 ) re: (Mi ) H and statement after Eq. (7), re: (M,),.l 5.3.3.3 Transmitting Current Response A secondary calibration of a projector can be ob
43、- tained by using a standard hydrophone to measure the generated sound field. The hydrophone must be locat- ed at sufficient distance, in the specified direction, for a spherically divergent sound field to exist at that spot. The apparent sound pressure at 1 meter then is deter- mined by multiplying
44、 the measured sound pressure at the remote point by the ratio of the distance d in me- ters at that point from the acoustic center of the pro- jector to the reference distance ( l meter). The trans- mitting current response is given by where Si = transmitting current response of the projector, hydro
45、phone, projector and the hydrophone, input terminals, hydrophone. erCf = open-circuit voltage output of the standard d = distance between the acoustic center of the , ip = current flowing at the projector electric (Me)ref = free-field voltage sensitivity of the standard The procedure is similar to t
46、hat described i n 5.3.3.3. The transmitting voltage response is given by where Se = transmitting voltage response of the emf = open-circuit voltage output of the standard d = distance between the acoustic center of the projector, hydrophone, projector and the hydrophone, electric input terminals, hy
47、drophone. ep = signal voltage applied to the projector (M, )rd = free-field voltage sensitivity of the standard 5.3.3.5 Sources of Error in Secondary Calibrations The sources of error in reciprocity calibrations (see 5.3.2.6) exist in a secondary calibration; however, few- er data are required in a
48、secondary calibration. For ex- ample, in the measurement of sensitivity, the projector current and the separation distance need not be mea- sured but must be the same when the standard and measured hydrophones are successively placed in the sound field. Accuracy and reliability can be increased by a
49、veraging the results of measurements with two or more standard hydrophones. 5.4 Measurement of Sensitivity and Response-Infrasonic and low Audio- Frequency Range 5.4.1 Coupler Reciprocity Calibration of a Hydrophone NOTE: If the projector obeys the electroacoustical reciprocity The absolute calibration of a hydrophone at low theorem, then its free-field voltage sensitivity can be determined by audio and infrasonic frequencies can be determine