Abstract:
Relative phase and sensitivity characteristics of individual transducer  eents in a long acoustic array are determined by sequentially positioning a transmitting element in predetermined spaced relation to each array element by means of an array holding member and a trough-like spacing member that contains a known transmission medium, driving the transmitting element with a test frequency input signal that is pulsed at a predetermined repetition rate, gating the output signal from the array element under test to an oscilliscope for phase comparison with an adjustably phase shifted version of the input signal, and measuring amplitudes of both input and output signals.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to testing and calibration of electroacoustic transducers, and more particularly to a novel method and apparatus for determining relative phase and sensitivity characteristics of elements of a long multi-element, high frequency array for the purpose of determining phase trimming and sensitivity shading necessary to achieve accurate near field focusing of the array. 
     In an ideal situation, measurements of phase and sensitivity parameters of the array receiving elements would be made while the elements of the array are simultaneously subjected to incidence of a high frequency, plane wavefront. Conventional methods have used a substantially plane wave produced by placing projectors in the far field region, by using collimators for converting a curved wave front to plane, or by using a projecting array having a plurality of projectors shaded so as to generate a substantially plane wave. The last method is described in U.S. Pat. No. 3,393,400 to W. J. Trott, assigned to the assignee hereof. 
     These prior art techniques, while of considerable use in certain situations, are subject to disadvantages, particularly when operating at frequencies in the megaherz ranges. Thus, for long, high frequency arrays the required distance between a source projector and the array is extremely large and creates many difficult problems of alignments and compensations for environmental constraints. Acoustic collimators and calibrating arrays of the type mentioned in the above noted patent are expensive and alignment is extremely difficult. 
     SUMMARY OF THE INVENTION 
     With the foregoing in mind, it is a principal object of this invention to provide an improved method and apparatus for measuring a variety of operational characteristics for each of a plurality of electroacoustic transducer elements of a long, multi-element array. 
     Another important object of the invention is the provision of a simple, reliable, inexpensive and accurate method and apparatus for obtaining comparative measurements of phase and sensitivity of side-by-side transducer elements of an acoustic array corresponding to those which would be obtained in response to a plane wave. 
     Yet another object of the invention is the provision of apparatus which permits testing and measuring of the desired parameters under controlled, repeatable conditions within limited working spaces. 
     As still another object, the invention aims to provide those advantages obtaining from avoidance of tests in open water, expensive anechoic chambers or tanks, use of complex collimators and other plane wave front generators. 
     Other objects and many of the attendant advantages will be readily appreciated as the subject invention becomes better understood by reference to the following detailed description, when considered in conjunction with the accompanying drawings. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side view, partly in elevation and partly in section, of a long, plural element acoustic array in association with testing and positioning apparatus forming part of this invention; 
     FIG. 2 is a sectional view, on an enlarged scale, of the array and apparatus of FIG. 1; and 
     FIG. 3 is a diagrammatic illustration, in block form, of apparatus embodying the invention and used in practice of the method thereof. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIGS. 1 and 2, a long, high frequency electroacoustic array is generally indicated at 10 and comprises an array holder or support member 12 formed of a rigid material such as steel. Member 12, the external configuration of which is generally rectangular, is provided with an upper rectangular recess 14 and a lower rectangular recess 16 so as to be substantially H-shaped in cross-section. A horizontal web 18 separates these recesses. 
     A plurality of electro-acoustic transducer elements 20a, 20b,--20k are disposed in sided-by-side relation in the upper recess 14 of member 12. Each of the elements 20a, 20b, 20k is provided with appropriate lead wires 22 which project through corresponding openings 24 in the web 18 into the lower recess 16. A body of waterproof, electrically insulating, and acoustically transparent potting compound 26 fills the recess 14 around the transducer elements 20a, 20b,--20k. 
     It will be understood at this point that the electro-acoustic transducers 20a, 20b,--20k may be designed for receiving only, but will usually be of the reversible or transmit/receive type. 
     Mounted on the upper surface of the array holder member 12 is a spacer member 30, formed of steel or other rigid material. Member 30 is generally rectangular in outer configuration and is substantially congruent with member 12. The member 30 has side walls defining a rectangular opening 32 through the member, which opening is congruent with the recess 14 in member 12. A pair of parallel ribs 34 extend inwardly from the sidewalls of member 30 to serve as a guide or track for a signal source in the form of a transmitting electroacoustic element 38. The transmitting element 38, which is served by suitable flexible lead wires 40, is adapted to be moved along the track or guide ribs 34 between successive operating positions opposite each of the array elements, 20a, 20b,--20k. 
     The spacer member 30 has its lower edge in tight engagement  with the upper edge of the holder member 12, and is fixed thereon as by bolts 42. The opening 32 is thereby closed by the surface of the potting 26 to define a cavity in which a suitable acoustically transmitting medium 46 having known acoustic properties is contained. In the embodiment being described the transmission medium is fresh water, although other fluids may, of course, be used. The opening or cavity 32 is filled with the transmission medium 46 sufficiently to ensure that the lower face of transmitting element 38 is in full contact therewith. A weight 38a is carried by the transmitting element 38 to ensure that it is held in good contact with the track ribs 34. 
     It is important that the upper surfaces of the track ribs 34 are as nearly parallel as is praticable to the upper surfaces of the array elements 20a, 20b,--20k. In a working embodiment of the invention wherein frequencies up to 1 MHz are used, the mating surfaces of the holder member 12 and the spaces member 30 were carefully machined flat and the upper track rib surfaces machined parallel thereto within 0.001 inch. This assures that the same spacing d will exist through a column of the transmission medium, between the transmit element 38 and the surface of the potting 26, for each of the array elements 20a, 20b,--20k. 
     It is also advantageous to have the width of the cavity 32 on the order of twice the vertical dimension thereof, to minimize interference from sidewall reflections. These may be further reduced by use of anechoic coatings (not shown) on the inner surfaces of the spacer member 30. 
     Referring now to FIG. 3, an oscillator 50 provides the operating frequency for the tests, and has its output connected as shown to a tone burst generator 52, to a frequency meter 54, for checking the output frequency of the oscillator, and to a variable phase shifter 56. The tone burst generator 52 serves to provide an output of only n cycles of the test frequency f every m seconds. This tone burst or pulse output of generator 52 is applied, as shown, to a monostable multivibrator or one shot 60 and to a low output impedance amplifier 62. 
     The amplifier 62 drives the transmit element 38 to project pulses of an acoustic signal 64 of n cycles, at the test frequency, every m seconds into the transmission medium 46, where: 
     n = d/λ, λ is the wavelength of frequency f in the transmission medium , and n ≦ m/4. This assures that there will be definite intervals, between transmission pulses, of sufficient length that no interference will occur between reflections from the potting or array element surfaces and a subsequent pulse. 
     The electrical output of the one of the array elements under test at any time is connected to the input of a fixed gain, inverting amplifier 68. The output of amplifier 68 is adapted to be passed by a gate 70, when enabled, to the x axis input of a first oscilliscope 72 and to a terminal 74. The gate 70 has its control input connected to the output of the one shot 60. The y axis input of the oscilliscope 72 is connected to the output of the phase shifter 56. 
     A second oscilliscope 76 has its y or vertical axis connectable alternatively, as by a contactor 78, to terminal 74 and to a terminal 80 connected to the output of the amplifier 62. 
     MODE OF OPERATION 
     With the oscillator 50 operating to provide a selected test frequency f, as measured by meter 54, the transmit element 38 is positioned along the track ribs 34 of spacer member 30 at a location directly opposite a selected one of the array elements 20a, 20b,--20k, say element 20a. The tone burst generator provides pulses of n cycles of alternating current at frequency f and a pulse repetition rate of m per second. These pulses are amplified by amplifier 62 and projected as acoustic energy waves 64 into the transmission medium. 
     The selected array element converts the acoustic energy to corresponding a.c. electrical signals of frequency f. These signals are inverted and applied to the gate 70. The one shot 60 enables the gate 70 during the pulse period that the n cycles are being transmitted by the transmit element 38, and disables the gate during other times. Thus, gate 70 is enabled m times per second to pass the inverted output of the array element to the x axis input of the oscillisope 72. Meanwhile, the oscillator output f is passed by phase-shifter 56 to the y axis input of the oscilliscope 72 so as to produce a Lissajous figure indicative of the phase relationship between the input of amplifier 62 to the transmit element and the output of the array element under test. The phase shifter 56 is then adjusted until the input to the transmit element and the output of the array element are in phase as indicated by the oscilliscope 72. At this time a reading is taken of the phase meter 76 which shows the amount of phase shift introduced by the phase shifter. This value is recorded for later comparison with subsequently acquired phase shift values for each of the other array elements. 
     Next, a peak-to-peak voltage amplitude reading is made of the input signal Vi to the transmit element 38. Then, contactor 78 is moved to terminal 74 and a peak-to-peak voltage amplitude reading is made of the output signal Vo from the array element being tested. The ratio of the output amplitude to the input amplitude provides a relative sensitivity value for the array element. 
     The foregoing procedure is repeated for each of the other array elements 20b,--20k, with the transmit element 38 positioned directly over each array element during the testing thereof. It will be appreciated that, inasmuch as the transmission medium column d is identical for each such position, any differences in the phase meter readings for the respective array elements will be due to phase change differences in the array elements themselves. Accordingly, the phase meter readings provide a measure of the relative phase change effects of the individual elements. 
     The relative phase and relative sensitivity values are readily used in a known manner, by introducing phase change delays and signal level adjustment for each of the array elements, as necessary, to achieve the desired near field focusing of the array for use either in a projecting mode or a receiving mode. 
     Various refinements of the invention and alternative constructions will be apparent to the skilled practitioner. For example, in order to maintain the transmission medium 46 at a constant level that will assure full contact with the transmit element, a reservoir and pumping system can be included that will replenish the medium automatically as necessary. Also, an indexing means could be provided for accurately and quickly repositioning the transmit element over each array element, and switching means provided so that a single oscilliscope, or equivalent device for the purpose, can be used for making the phase comparisons and amplitude measurements. 
     Obviously, other embodiments and modifications of the subject invention will readily come to the mind of one skilled in the art having the benefit of the teachings presented in the foregoing description and the drawing. It is, therefore, to be understood that this invention is not to be limited thereto and that said modifications and embodiments are intended to be included within the scope of the appended claims.