# Bar-Wave Calibration of Acoustic Emission Sensors
## 3 Results and Discussion
### Further Discussion  
Results given above lead to the following observations.  
1. Receiving sensitivities to bar waves and to normally incident waves are different for any given sensor.
2. Differences in the sensitivities to bar waves and to normally incident waves increase with the increase in frequency above approximately 200 kHz with the bar-wave sensitivity becoming lower at higher frequencies. In the low frequency range, the opposite is usually observed; that is, the bar-wave sensitivity is higher.
3. The differences at higher frequencies noted in b. above are smaller when the sensing area of the sensor is small, as in KRN and Pinducer sensors. In contrast, the differences are larger for larger sensors, like V101 and V103.
4. Even when spectral flatness exists for the receiving sensitivity to normally incident waves, the receiving sensitivity to bar waves exhibits variable spectral responses. See for example, Figures 11, 16 and 20. For small aperture sensors like KRN and Pinducer, however, a limited range of flat bar-wave sensitivity does exist.  
These observations are generally consistent with aperture effect or effect of waveform cancellation over the sensing area of a sensor, as discussed by [PERSON] and others [2; 14; 15; 16]. The basic concept is that when a full-cycle wave covers the sensing area, the sensor output is minimal due to cancellation. The lowering of bar-wave sensitivities at higher frequencies is indeed consistent with the waveform cancellation concept in principle. The models considered so far have treated the surface wave cases with straight or circular wavefront and the wave velocity is constant. In the case of bar waves, many modes coexist and distinction between the modes is difficult. This difficulty also applies to the spectral dips observed. For sensors of the same size, say 12.7 mm in diameter, or V103, R6a, R15, and R15a, frequencies corresponding to the clearly observed dips are reasonably consistent. In many cases, dips were present at 350-500 kHz, and these can match the aperture effect prediction if one uses wave velocities within 10-15% of those calculated by a finite element method [9].  
With respect to the aperture effects discussed above, it is important to remember that bar-wave velocities of various modes depend on the size of the bar used. For a given application, it is essential to  
Figure 22: Receiving sensitivity of Olympus V101. The displacement receiving sensitivity to bar waves is shown in solid blue curve. Green curve is the displacement receiving sensitivity to normally incident waves from reference [4]. The velocity sensitivity corresponding to the solid blue curve is given in dotted blue curve.  
recognize the size effects in predicting where spectral dips occur. However, such effects are expected to be proportional to bar thickness (and width to a lesser extent), estimation is feasible without recalculating the dispersion curves.  
In regard to the length of a bar needed for the present method, it is unnecessary to use more than 1 m length. Using the same L value of 300 mm, the reflection from the far end travels 1.4 m or travel time of 230 us (with aluminum or steel bars). This is adequate when the slowest velocity to be evaluated is 1.5 mm/us, as was the case in the present study. This shortened length gives portability when laser interferometry has to be conducted elsewhere or for taking the set-up to field test sites.