# Validation of Acoustic Emission Waveform Entropy as a Damage Identification Feature
## 3 Materials and Methods
### Experimental Evaluation Procedure
#### 3.2.2 Performance Against Hit Definition Time  
A realistic AE waveform generated in metal was used to evaluate the performance of AE entropy against the hit definition time (HDT) [39]. In this test, the same metal plate mentioned in the previous section was excited with a Vallen VS-900 AE transducer to generate elastic waves. The Vallen VS-900 transducer can be used both as an AE source and as a sensor. In this test, one AE transducer was used as a source to generate elastic waves and a second AE transducer was used as a sensor to measure the elastic waves.  
Rectangular pulses with a very small width applied to the AE source transducer were used to generate the elastic waves. The rectangular pulsing pattern shown in Figure 6 was used to introduce a precise delay between each of rectangular pulses (i.e., 240 \(\upmu\)s between the first and second pulses; 375 \(\upmu\)s between the second and third pulses). The delay was used to separate the generation of an elastic wave in the plate, which supported the investigation into the influence of HDT in the recorded waveform. To vary the amplitude of the elastic wave in the plate, different heights of rectangular pulses were chosen (height of the first and second pulses were 0.5 and 0.75 times the third pulse). The elastic wave was captured as an analogue signal, with another AE sensor attached to the metal plate. The total distance between the transducer was 130 mm. Figure 7 illustrates the transducer arrangement used in the test. The narrow rectangular pulses act like PLB impulse excitation in the plate. Therefore, the frequency response of the elastic wave in the plate was not known prior to the experiment. The Vallen VS-900 was chosen as a transducer for both the pulsing and capturing of elastic wave as it was sensitive to a wide range of frequency and can successfully capture the true nature of the elastic wave. The transducers used in this experiment were the same as the one used as a sensor in the previous experiment (i.e., in Section 3.2.1). Figure 4 shows an image of the sensor and its frequency sensitivity spectrum.  
The analogue signal captured by the sensor was passed on to three different channels in AMSY-6. Each channel acquired the signal with a different HDT than the other. Table 3 shows the range of HDTs used for this analysis. The rest of the acquisition settings were the same as the ones in Table 2. The traditional AE features for each channel were calculated by the AE data acquisition system, and AE entropy was calculated from the discrete voltage value using a MATLAB MATLAB(tm) script written for this purpose. Finally, the influence of HDT on AE entropy was evaluated and compared with the traditional features.  
\begin{table}
\begin{tabular}{c c} \hline
**Hit Definition Time (HDT)** & **80 \(\upmu\)s** \\ \hline Re-Arm Time & 1 ms \\ Sampling Frequency & 5 MHz \\ Max duration TR window length & 820 \(\upmu\)s \\ Frequency Filter & 95k Hz–850 kHz \\ Threshold & 60 dB \\ \hline \end{tabular}
\end{table}
Table 2: Acquisition settings in all the channels.  
Figure 6: Rectangular pulsing pattern for exciting the source transducer.