# Bar-Wave Calibration of Acoustic Emission Sensors
## 3 Results and Discussion
### AE Sensor Response  
#### 3.2.1 KRN Sensor  
Two KRN sensors (KRNBB-PC #11053 and #11063: with a field-effect transistor (FET) buffer) were positioned at L = 300 mm with the output connected to a preamplifier/power supply (KRN AMP 1 BB-J). These were made by KRN Services, Richland, WA, USA. The preamplifier section was not used since it was saturated. These sensors have a small diameter of 1 mm sensing area, approximating a point-contact sensor. The output waveform (#11063) and its CWT spectrogram are shown in Figure 6. Overall, this resembles Figure 4 and all the peaks (cw1 to cw6) appeared at the corresponding positions. Extra traces are also visible as well. In Figure 6, however, strong spectral intensities exist below 50 kHz unlike in Figure 4. This is the zone where bar-wave modes are those of x- and y-displacement dominated modes, seen in Figure 5a. This low-frequency response was, in part, a result of FET use since KRN sensors without FET showed low spectral intensities below 100 kHz (see Appendix B, Figure A4 in particular). As will be discussed later in Appendix D, the use of a coaxial cable and 10 k\(\Omega\) termination of the non-buffered sensors also contributed to reduced sensitivities at lower frequencies below 300 kHz. The appearance of cw6 in Figures 4 and 6 was different from other peaks as it showed a constant wave velocity. By placing a KRN sensor on the top or bottom surface of the test bar, the observed waveforms were compared, as plotted in Figure 7. Starting from the wave arrival at 52 \(\upmu\)s, most of the peak positions of the waveforms detected at the two surfaces (blue: top surface, red: bottom surface) matched well to 150 \(\upmu\)s (shown in Figure 7a,b). That is, the wave modes are symmetric to 150 \(\upmu\)s. Consequently, the position of cw6 is still where the symmetric modes are dominant and this peak is likely to be a combination of multiple modes, dominated by the z displacement. The possibility of this peak to be from asymmetric plate modes or from the SH mode can be discarded. (Appendix A also shows that asymmetric wave excitation did not produce this peak, further supporting its origin to be of symmetric modes.) As shown in Figure 7c, the two curves diverged beyond approximately 150 \(\upmu\)s, implying the dominance of slow asymmetric modes at wave velocity below 2 mm/\(\upmu\)s. This means asymmetric modes are produced even with the use of a symmetric wave excitation method. Sensitivity of propagated plate wave signals to the position of displacement input was already recognized by [PERSON] et al. (2010) in their three-dimensional finite-element modeling of plate waves. Receiving sensitivity was obtained by subtracting the center-point displacement magnitude spectrum from the output-voltage magnitude spectrum of a KRN sensor, in the same manner as in the previous studies [4; 5]. Results are given in Figure 8a for #11063. Here, the displacement receiving sensitivity is given in dB in reference to 0 dB at 1 V output per 1 nm of normal displacement, shown in blue curves. Solid line is for the bar-wave data, while dashed blue curve is the displacement receiving sensitivity to normally incident waves. The latter was determined using Olympus V192 as a transmitter [4]. Note that this KRN sensor has the peak sensitivity level of about 15 dB higher than those without FET (reported in Figure 6c of reference [4]) even though the latter was measured using 100 M\(\Omega\) input impedance of a 100\(\times\) probe. (More sensitivity loss (of approximately 10 dB) occurs when a 60 cm long microdot cable is used between the FET-less KRN sensor and digital oscilloscope input, as will be discussed in Appendix B). The bar-wave sensitivity was highest at 22 kHz and decreased 30 dB to 300 kHz, above which it was relatively flat to 1.4 MHz. The normally incident wave sensitivity was flat within \(\pm\)4 dB up to 1.2 MHz and differed most from the bar-wave sensitivity at frequencies below 100 kHz. The averaged normally incident wave sensitivity observed here was 1.14 dB between 22 kHz and 1.2 MHz.