## 7 Conclusions  
Split-spectrum processing with raised cosine filters of constant frequency-to-bandwidth ratio (FBR-RC-SSP) was proposed to improve the signal interpretation accuracy of L(0,2) ultrasonic guided wave (UGW) testing. The chirplet transform for time-frequency analysis was used to study the dispersion and multi-mode of the synthesized UGW signal (L(0,1) + L(0,2) + F(n,1) + F(n,3), \(1\leq n\leq 4\)). The signal-to-noise ratio gain (SNRG) and defect-to-coherent noise gain (DCRG) were introduced to reveal the effects of filter parame  
Figure 17: Experimental UGW signals before and after applying GS-SSP and FBR-RC-SSP (1 \(\leq\) defect number \(\leq\) 3, 60\({}^{\circ}\)\(\leq\alpha\leq 300^{\circ}\)). \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & \multicolumn{2}{c}{**GS-SSP-PT**} & \multicolumn{2}{c}{**GS-SSP-PTM**} & \multicolumn{2}{c}{**FBR-RC-SSP-PT**} & \multicolumn{2}{c}{**FBR-RC-SSP-PTM**} \\ \cline{2-9}  & **SNRG(dB)** & **DCRG(dB)** & **SNRG(dB)** & **DCRG(dB)** & **SNRG(dB)** & **DCRG(dB)** & **SNRG(dB)** & **DCRG(dB)** \\ \hline
1–60\({}^{\circ}\) & 16.27 & - & 18.15 & - & 18.79 & - & 20.82 & - \\
1–120\({}^{\circ}\) & 15.25 & 15.04 & 15.92 & 15.66 & 17.23 & 17.81 & 18.83 & 18.27 \\
1–180\({}^{\circ}\) & 14.31 & 15.21 & 17.71 & 17.56 & 19.45 & 19.11 & 20.20 & 20.05 \\
1–240\({}^{\circ}\) & 15.16 & 15.06 & 16.55 & 16.43 & 17.21 & 17.95 & 18.86 & 18.46 \\
1–300\({}^{\circ}\) & 13.87 & 13.31 & 14.19 & 13.54 & 16.59 & 16.14 & 18.39 & 18.40 \\
2–300\({}^{\circ}\) & 14.59 & 15.20 & 15.78 & 15.01 & 19.25 & 18.65 & 18.89 & 18.51 \\
3–300\({}^{\circ}\) & 14.12 & 14.06 & 16.65 & 16.58 & 20.81 & 20.28 & 22.97 & 22.16 \\ Mean & 14.79 & 14.64 & 16.42 & 15.79 & 18.48 & 18.32 & 19.85 & 19.31 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: SNRGs and DCRGs of experimental UGW signals processed by FBR-RC-SSP. ters on SNRG and DCRG, and the processing effects of FBR-RC-SSP were verified by the simulated and experimental UGW signals. Unlike the Gaussian (GS) filters, the raised cosine (RC) filters have transition bands in the shape of a truncated raised cosine cycle, and the energy contained in the RC filter is mostly concentrated around the center frequency for the flat top design. In addition, FBR-RC filters have a wider bandwidth compared with those of RC filters and the resolution of time pulse is higher. It is therefore easy to foresee that FBR-RC filters can better meet the requirements of minimizing the degree of overlap without losing substantial information compared with GS filters. In order to obtain the best processing effect, it is necessary to choose the appropriate filter parameters (total bandwidth \(B\), transition bandwidth \(B_{t}\), flat top width \(B_{r}\), filter spacing \(\Delta f\)) for the parameter sensitivity of SSP. Only defect and end-reflected echoes were retained in the synthesized, simulated and experimental UGW signals processed by FBR-RC-SSP, while the coherent noise was eliminated, and the signal interpretation accuracy was improved. In addition, Polarity Thresholding (PT) and Polarity Thresholding with Minimization (PTM) were the suitable signal recombination methods for L(0,2) UGW testing. In this way, the average SNRG and DCRG of FBR-RC-SSP were about 22.92% and 23.71% higher than the traditional GS-SSP, which may be meaningful to the further location and quantitative analysis of defect. Conceptualization, Y.W. and B.T. ; methodology, B.T. ; software, B.T. ; validation, B.T., A.C. and Y.W. ; formal analysis, J.X. ; investigation, Y.Z. and J.X. ; data curation, Y.Z. ; writing--original draft preparation, B.T. ; writing--review and editing, Y.W. and B.T. ; supervision, Y.W. ; project administration, Y.W. ; funding acquisition, Y.W. All authors have read and agreed to the published version of the manuscript. This research was funded by the Naval Engineering University, grant number 417214390. The authors would like to acknowledge the Nondestructive Testing Laboratory of Naval Engineering University School of Power Engineering for equipment support.