# Sound Field Modeling Method and Key Imaging Technology of an Ultrasonic Phased Array: A Review
## 3 Imaging Technology
### Nonlinear Ultrasonic Array Imaging
#### 3.3.1 Harmonic Imaging
It was originally mainly used for acoustic second harmonic imaging in the medical field, which can effectively extract the second harmonic time-domain signal, enhance the amplitude of the second harmonic, and suppress the odd harmonic components mainly generated by the experimental system [105]. Pulsed inversion nonlinear ultrasonic array imaging successively emits positive and negative pulses to obtain the respective A-sweep signals and the signals are sent to the buffer without post-processing after passing through the band-pass filter. At this time, the buffered positive pulse beam is summed with the negative pulse beam to achieve the inverted pulse signal processing. The subsequent signal processing process is identical to that of filter-based nonlinear imaging detection. [PERSON] et al. [106] of Peking University used the non-linear ultrasonic phased array NDT system (Figure 12) to compare and test the detection effects of conventional linear ultrasonic imaging, filtering based nonlinear ultrasonic imaging and pulse inversion nonlinear ultrasonic imaging for tungsten filament phantom. The ultrasound images of the tungsten filament phantom obtained by the three imaging methods are shown in Figure 13. It can be found that the horizontal and axial spatial resolution of pulse inversion nonlinear ultrasonic imaging (PI-NLI) is significantly improved compared with the other two methods. Figure 11: Experimental configuration of a closed-crack imaging method using subharmonic waves. (A is the center position of the LiNbO\({}_{3}\) single crystal, B is a passing point on the surface from A to the focal point C, which changes during scanning, and D is the position of an element of the array sensor. A–D is used to calculate propagation time in the specimen and wedge, and a digital bandpass filter is used to extract the fundamental (f) and subharmonic (f/2) components). The amplitude modulation technique is to transmit two pulses with different amplitudes and the same phase successively; for example, the latter can be set to be \(1/4\) of the amplitude of the former one, and then the echo signal generated by the second transmission is multiplied by four and subtracted from the echo generated by the first transmission. Since the amplitude of the second emission is much smaller than the first one, the nonlinear effect is smaller and the harmonic components are not obvious. The fundamental component is canceled and the harmonic component is preserved. Researchers have carried out a large number of nonlinear ultrasonic phased-array imaging detections based on the pulse amplitude modulation method. In [107] a nonlinear ultrasonic imaging method is proposed. This method analyzes the relationship between diffused sound energy, parallel focusing and sequential focusing, and the nonlinear effect under two different phase modes. The results show that the difference of the acoustic energy of two-phase modes within the excitation bandwidth (fundamental wave) can be used as the characteristic parameter for the acoustic energy of the two-phase modes. The results show that the difference of the acoustic energy of two-phase modes within the excitation bandwidth (fundamental wave) can be used as the characteristic parameter for the acoustic energy of the two-phase modes within the excitation bandwidth (fundamental wave). Figure 12: The principal block diagram of the nonlinear ultrasonic arrays NDT system. Figure 13: Ultrasonic images of a tungsten filament phantom using conventional ultrasonics/filtering–based nonlinear imaging/pulsed inversion nonlinear imaging methods). nonlinear imaging. Based on the nonlinear imaging method using ultrasound arrays in different phase modes, [PERSON] et al. (2018) studied the influence of the time delay and time window for diffuse field sound energy and nonlinear imaging, and the best parameters for nonlinear imaging were obtained when the time delay was 0.67 ms and the time window width ranged from (0.02 ms-0.16 ms).