## 3 Laboratory Testing
The increments of SNR and peak sidelobe level of 5-bit Barker code are better than that of 3-bit Barker code. The echo SNR is improved by 2.6 dB (FE) and 1.2 dB (BE), respectively. The peak sidelobe level is decreased by 2.8 dB (FE) and by 4.9 dB (BE). As the code length increases, 7-bit, 11-bit and 13-bit Barker codes have a significant improvement in SNR. The average increment of SNR for 7-bit, 11-bit and 13-bit Barker codes are 21.7 dB, 23.6 dB and 24.2 dB compared to that of sine wave, respectively. However, the peak sidelobe level of these codes has not significantly improved or even dropped. The reason for this may be influenced by the overlength of the bark code. Table 3 shows the increased SNR of Golay code pairs of different lengths at 30 kHz. Due to the Golay code pair can eliminate side lobes, Table 3 only shows the SNR of different lengths Golay code pair. The increased SNR rises in lockstep with the length of the transmitted Golay code pair increased, as predicted by the theory. The values of the increased SNR of Golay code pairs from the front and back pipe ends are almost the same in the experiment. From experimental results, the length of Golay code pair is increased exponentially, the improved SNR should increase with an approximate 3 dB increment that is consistent with the theoretical analysis results. When the sequence length of Golay code pair is 16, the improved SNR reached its maximum value of 28.3 dB. Table 4 shows the increased SNR and PSL of the 3-bit Barker code convolution 4-bit Golay code pair at different frequencies. From the experimental data, it can be found that the frequency of 40 kHz is the best operational frequency to excite the torsional wave mode T(0,1) using the BCG code pair for pipe inspection. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Frequency** & **Front SNR (dB)** & **Front PSL(dB)** & **Back SNR (dB)** & **Back PSL (dB)** \\ \hline
30 kHz & 28.9 & \(-13.3\) & 30.1 & \(-10.5\) \\
35 kHz & 31.4 & \(-27.9\) & 32.4 & \(-24.3\) \\
**40 kHz** & **32** & \(-28.5\) & **33.5** & \(-19.5\) \\
45 kHz & 29.3 & \(-22\) & 29.8 & \(-22.9\) \\
50 kHz & 26.7 & \(-21.7\) & 28.1 & \(-16.9\) \\ \hline \hline \end{tabular}
\end{table} 
Table 4: The SNR and PSL of b3g4 code from FE and BE at different frequencies. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Code Bit** & **Front SNR (dB)** & **Front PSL (dB)** & **Back SNR (dB)** & **Back PSL (dB)** \\ \hline
3 & 16.7 & \(-11.8\) & 19.1 & \(-13.3\) \\
5 & 19.3 & \(-14.6\) & 20.3 & \(-18.2\) \\
7 & 20.9 & \(-18.7\) & 22.4 & \(-20.8\) \\
11 & 22.9 & \(-18.2\) & 24.3 & \(-21.2\) \\
13 & 23.6 & \(-19.1\) & 24.8 & \(-22\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The SNR and PSL of Barker codes of different lengths at 30 kHz from the front and back pipe ends. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Code Bit** & **Front SNR (dB)** & **Front PSL (dB)** & **Back SNR (dB)** & **Back PSL (dB)** \\ \hline
2 & 18.2 & 19.4 \\
4 & 21.2 & 22.4 \\
8 & 24.3 & 25.5 \\
16 & 27.2 & 28.3 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The SNR of Golay code pair of different lengths from the FE and BE at 30 kHz. The increment of echo SNR to the processed BCG code pair signal is 28.9 dB (FE) and 30.1 dB (BE) at 30 kHz compared to sine wave and the peak sidelobe level is \(-\)13.3 dB (FE) and \(-\)10.5 dB (BE). Since the FE and BE echoes are basically similar, the SNR and peak sidelobe level are also similar after echo signal processing. The increment of SNR of the code at 35 kHz is 2.5 dB (FE) and 2.3 dB(BE) higher than that at 30 kHz. The peak sidelobe level of the processed signal at 35 kHz is \(-\)27.9 dB (FE) and \(-\)24.3 dB (BE), respectively. The echo signal at 40 kHz shows the best performance among all coded signals. The increment of SNR at 40 kHz is 32 dB (FE) and 33.5 dB (BE) and the sidelobe level is \(-\)28.5 dB (FE) and \(-\)19.5 (BE). The performance of processed signal decreased at 45 kHz and 50 kHz.