# Acoustic Emission Wave Velocity Measurement of Asphalt Mixture by Arbitrary Wave Method
## 5 Analysis and Discussion
### Discussion  
By comparing and analyzing the initial wave velocity and corresponding AE signals measured by the three methods, it can be concluded that there are differences in wave velocities and signal characteristics between different methods. Table 4 shows the comparison and analysis results of the three AE wave velocity measurement methods.  
For the PLB method, which is the most widely used method at present because the lead breaking is performed artificially, the AE events produced by each lead breaking are different. Different AE events correspond to different waveforms, so it is necessary to modify the threshold of each lead-breaking waveform when calculating the wave velocity. As can be seen from Table 1, the threshold correction value of each lead breaking is different, which is also the reason for the large dispersion of the initial AE wave velocity measured by the PLB method. At the same time, the attenuation of the AE signal obtained by the PLB method is large, and its frequency is difficult to distinguish from the noise signal, so it will seriously affect the accuracy of the wave velocity measurement results.  
The AE in the AST method is excited by the same sensor, so the AE events generated are the same. Therefore, in the calculation of the wave velocity, only one AE waveform needs to be threshold corrected, and this threshold correction value is applicable to all AE events in the AST method. So, the initial wave velocity measured by the AST method has little dispersion. However, because the pulse signal excited by the excitation sensor is very strong, the signal will crosstalk to other channels, resulting in the signal collected by the receiving sensor containing a strong crosstalk signal. Therefore, before the first arrival time of the AE wave is determined, the waveform of the strong crosstalk signal needs to be blanking processed. Since the pulse intensity of each excitation is random, the intensity of the crosstalk signal is different, so the blanking time also needs to be determined for each AE event. Therefore, the AST method is not the best method to measure the AE wave velocity.  
In the AW method, the AE signal is generated by the excitation of the modulated AW (sine wave), so there will be no crosstalk signal. In addition, each AE event generated by the AW method is the same, so only one AE signal waveform needs to be threshold corrected. Therefore, the initial wave velocity measured by the AW method has very little dispersion. At the same time, the attenuation of the AE signal obtained by the AW method is small, and the frequency of the AE signal and noise signal is easy to distinguish, which  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Methods** & **PLB** & **AST** & **AW** \\ \hline Discreteness & large & small & small \\ Signal types & burst & burst & continuous \\ Degree of attenuation & large & small & small \\ Time-domain waveform & unfixed & fixed & fixed \\ Frequency distribution & unfixed & fixed & fixed \\ Frequency characteristics & high frequency and & high frequency and & high frequency and \\ of AE & small amplitude & small amplitude & large amplitude \\ Frequency characteristics & low frequency and & low frequency and & low frequency and \\ of noise & large amplitude & large amplitude & small amplitude \\ \hline \hline \end{tabular}
\end{table}
Table 4: Comparison results of three wave velocity measurement methods.  
is conducive to determining the first arrival time of the AE wave and the calculation of the AE wave velocity. Therefore, the AW method has great advantages as a method of measuring the AE wave velocity.