## 4 Results and Discussion
### Actual Detection of Debonding Defect Based on Air-Coupled Ultrasonic Testing  
According to the analysis in Section 4.2, based on the finite simulation results, the air-coupled ultrasonic transmitted longitudinal wave T1 can effectively characterize debonding defect, but the effect is unsatisfactory in actual quality evaluation of honeycomb sandwich composite. This is because the ultrasound needs to pass through multiple air-solid coupling interfaces during actual detection, and will be affected by a variety of noise, resulting in serious energy attenuation, high noise level, poor signal-to-noise ratio, and the transmission initial wave cannot be identified (as shown in Figure 10). Therefore, the received signal of Figure 10 could not be used to evaluate the quality of honeycomb sandwich composite. Figure 8: The sound field distribution of a intact honeycomb sandwich composite model and a honeycomb sandwich composite model with embedded debonding defect at 20.01 μs; (**a**) The sound field distribution of an intact honeycomb sandwich composite model at 20.01 μs; (**b**) The sound field distribution of a honeycomb sandwich composite model with an embedded debonding defect at 20.01 μs. R: reflected longitudinal wave; El: edge longitudinal wave; T1: transmitted longitudinal wave; T2: transmitted longitudinal wave; LL: leakage Lamb wave; L: Lamb wave. Figure 9: A-wave signal obtained from the computational simulation of the intact model and the embedded defect model. (**a**) corresponds to the intact model, and (**b**) corresponds to the embedded defect model. In order to overcome these problem, pulse compression technique in radar field is introduced into ACUT system. The advantage of this signal processing technology is increase the average transmitted power while still maintaining the range resolution [26; 27; 28; 29]. When pulse compression technique is applied to ACUT system, the problem of poor SNR and weak transmission signal can be significantly improved. This technique is specifically performed by a matched filter, which correlates the received signal with the reference signal. In actual processing, to improve the computational efficiency of correlation algorithm, the pulse compression is usually implemented in frequency domain. The implementation steps are as follows: first, the time-reversed function of reference signal r(t) is processed by the Fast Fourier transform (FFT); second, the Fast Fourier Transform is implemented on received signal z(t); then, the two transformed signals are multiplied in frequency domain; lastly, the multiplication result is processed by Inverse Fast Fourier transform (IFFT), and finally the compressed signal o(t) is output. The above process can be expressed by Equation (5):  
\[\mathrm{o(t)}=\mathrm{IFFT}\{\mathrm{FFT}\{\mathrm{r(-t)}\}\cdot\mathrm{FFT}\{ \mathrm{y(t)}\}\} \tag{5}\]  
According to the modulation information of ultrasonic signal, this signal processing technique can be divided into phase modulation pulse compression technique (e.g., Barker-coded pulse compression) and frequency modulation pulse compression technique (e.g., non-linear chirp pulse compression technology). This paper focuses on the application of P3 polyphase coded pulse compression technique. The phase angle of P3 polyphase code signal can be expressed as:  
\[\theta_{\mathrm{n}}=\frac{\pi(\mathrm{n}-1)^{2}}{\mathrm{N}^{2}},\mathrm{n}= 1,\ldots,\mathrm{N}^{2} \tag{6}\]  
where N is the number of bit. When a value of N is given, the P3 polyphase coded sequence with specific phase angle can be generated.