# Bar-Wave Calibration of Acoustic Emission Sensors
## Appendix B KRN Sensors without FET  
Two KRN sensors (KRNBB-PCP #12035 and #12060: without an FET buffer) were tested at L = 300 mm with the output termination (or input resistance R\({}_{\text{in}}\)) of 10 k\(\Omega\) along with a microdot cable of 60 cm length (C = 72 pF). The input capacitance of the oscilloscope (14 pF) is also in parallel. These have a small diameter of 1 mm sensing area, as in the FET-buffered version. The output waveform (#12035) and its CWT spectrogram are shown in Figure 14. Overall, this resembles Figure 4, but spectral intensities below 100 kHz are absent unlike in Figure 6. This part differs from the corresponding part of FET-buffered KRN sensors discussed earlier. Receiving bar-wave displacement sensitivity was obtained in the same manner as in the main text. Results are given in Figure 10 for #12035. Red curve gives the sensitivity with the same set-up as in Figure 11; that is, with C = 72 + 14 = 86 pF and R\({}_{\mathrm{in}}\) = 10 k\(\Omega\). Blue curve is the data for C = 10 pF and 10 M\(\Omega\) (with the use of a BK Precision 10\(\times\) probe) and green curve for C = 86 pF and R\({}_{\mathrm{in}}\) = 100 k\(\Omega\). The blue and green curves are separated by 10 dB, while the green and red curves match above 300 kHz. Below 300 kHz, the red curve had smaller increase of 18 dB in comparison to 30 dB rise observed in the blue and green curves. All three curves were relatively flat from 300 kHz to 1 MHz except for many fluctuations. The sensitivities further decreased at higher frequencies. For the FET-buffered sensors (cf. Figure 8b), comparable sensitivity increases at low frequencies were found and general trends are similar to the cases of high R\({}_{\mathrm{in}}\) (blue and green curves). The receiving sensitivities to bar waves (in solid curves) and normally incident waves (in dashed curves) are compared in Figure A6 for #12035. The data was determined with the output termination of 10 k\(\Omega\) via a coaxial cable (with 72 pF capacitance). In comparison to the corresponding data for the FET-buffered sensors given in Figure 8a, this figure shows general reduction in sensitivities. For the peak values of the blue dash curves, and for the average levels over 300-900 kHz of the blue curves, the differences are about 25 dB. The decrease in the displacement sensitivity below 100 kHz (blue dash) was even higher. Previously, the sensitivity data to normally incident waves was obtained in reference [4], using a 100\(\times\) probe in order to minimize sensor loading. This corresponded to R\({}_{\rm in}\) = 100 M\(\Omega\) in parallel to 3 pF. The peak value for the high-impedance data was \(-10\) dB (Figure 6c in reference [4]), shown here as green curve. For the FET-buffered sensor, it was 4.7 dB, as was shown in Figure 8a (blue dash curve). This sensitivity difference of 15 dB comes from the amplification effect of the FET. For the blue dash curve of Figure A6, the receiving sensitivity was reduced by 8 to 12 dB by lowering the input resistance to 10-k\(\Omega\) in parallel to 72 pF. A part of this loss (7.3 dB) was due to the cable capacitance as the sensor capacitance is 40 pF (33 pF for #12060 and the loss is 8.1 dB). Some loss also comes from the reduction of the input impedance, and from the input capacitance of digital oscilloscope (14 pF). Thus, the use of a coaxial cable contributes a 10 dB loss, corresponding to the difference between blue and green curves in Figure A5. Figure A7 gives CWT spectrogram corresponding to Figure A4, except asymmetrical excitation method was used. There are peaks at cw1 to cw5, but cw6 is absent. Instead, a prominent peak appears at 140 us and 700 kHz, as was the case for Figure A2. This is the only peak associated with the asymmetrical excitation, although the mode is unknown. The peak at cw3 is much stronger, and it stretched to higher frequency side, to 650 kHz. There was a weak trace in Figure A4 at this position, although its origin is unknown. As noted for Figure A2, many scattered spot intensities were seen.