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Reciprocity calibration reciprocity applies to a

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Reciprocity Calibration Reciprocity applies to a category of passive electromechanical transducers that have two important characteristics: (1) they are purely electrostatic or purely electromagnetic in nature and (2) they are reversible (can be used as either a source or a receiver of mechanical energy). This category includes all known commercial acoustic emission transducers without preamplifiers. For such a transducer, reciprocity relates its source response and its receiver response in a specific way. If two exactly identical transducers are used, one as a source and one as a receiver, both coupled to a common medium, and if the transfer function or Green’s function of the medium from the source location to the receiver location is known, then from purely electrical measurements of driving current in the source and output voltage at the receiver, the response functions of the transducers can be determined absolutely. With nonidentical transducers, three such measurements (using each of the three possible pairs of transducers) provides enough information to determine all of the response functions of the transducers absolutely. The primary advantage of the reciprocity calibration technique is that it avoids the necessity of measuring or producing a known mechanical displacement or force. All of the basic measurements made during the calibration are electrical. It is important to note, however, that the mechanical transfer function or Green’s function for the transmission of signals from the source location to the receiver location 58 Acoustic Emission Testing
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must be known. This function is equivalent to the reciprocity parameter and is the frequency domain representation of the elasticity theory solution mentioned in the discussion on the step function force calibration. The application of reciprocity techniques to the calibration of microphones, 61-66 hydrophones 67 and accelerometers 68 is well established. The reciprocity technique was proposed in 1976 for the calibration of acoustic emission transducers coupled to a solid and was subsequently implemented by one steel producer as a commercial service. 52,69,70 One steel producer’s calibration facility has used a cylindrical steel test block 1.1 m (44 in.) in diameter by 0.76 m (30 in. ) long to perform rayleigh calibration (analogous to surface calibration) and P wave calibration (analogous to through-pulse calibration). In the rayleigh calibration, the transducers are separated by 0.2 m (8 in.) on the same surface of the block; for the P wave calibration, the transducers are on opposite faces on epicenter. The technique uses essentially continuous wave measurements but the signals are gated to eliminate reflections from the block walls. For a set of three transducers, the three electrical voltage transfer functions and the electrical impedances of all transducers are measured. From these data, receiving response (in volts of output per meter per second of input) and source response (meters per second of output per volt of input) are calculated for the range of 100 kHz to 1 MHz. This
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  • Fall '19
  • Nondestructive testing, Acoustic Emission, Acoustic Emission Testing

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