# A Review of Acoustic Emission Source Localization Techniques in Different Dimensions
## 3 Sound Source Localization Methods for Two-Dimensional (2D) Structures
### Beam Forming  
Another classical method for sound source localization is beam forming. It is a wireless communication technology that employs a sophisticated strategy during data transmission. The core principle is that the transmitting end will weight the data to be sent and then form a focused narrow beam, which is like a laser beam and directly points towards the target receiving user. The main purpose of doing so is to enhance the signal strength sent to the target user, reduce signal attenuation during propagation and significantly improve the demodulation signal-to-noise ratio of the target user, especially for users located at the edge of the cell with relatively weak signals. This technology can significantly improve their data throughput. The beam forming method has been successfully applied in multiple fields such as sonar, telecommunications, radar, sound source recognition and localization. Beam forming was first proposed by [PERSON] for sound source localization. The advantages of constraint formation technology are high positioning accuracy and robustness. The limitations are (1) that it requires a large number of sensor groups; (2) when comparing the shape of signals in beam forming, all sensors in the array should have similar amplitude and phase responses; and (3) to avoid spatial aliasing, the beam forming array needs to have a relatively small aperture. [PERSON] et al. (2018) proposed an improved beam forming approach for civil engineering structures, as shown in Figure 5. [PERSON] et al. (2020) realized sound source localization in plate structures by improving the beam forming method, examined the effect of AE characteristics on the localization accuracy and proposed a method for determining the localization speed by combining the plate wave theory and wavelet transform, as shown in Figure 6a. Figure 5: Four-sensor beam forming technique. [PERSON] et al. [23] compared the execution time of various time-domain and frequency-domain beam forming techniques on general-purpose processors as well as their hardware requirements. [PERSON] et al. [24] proposed a new method working in low-frequency combined with high-frequency secondary AE beam forming localization for internal defects in thin plates. The signal source position using high- and low-frequency bands was first determined by analyzing the main flap and sidelobe parameters with the maximum sidelobe level (MSL), then the low-frequency localization of the signal envelope was used to determine the range of the main flap and, finally, signal filtering was performed for high-frequency localization through the narrowing of the scanning scope by the determined range of the main flap. The results showed that the method avoided the beam formation of the main flap being wider and the existence of influence of the side flap, and the defects in thin plates were accurately localized, but it also solved the linear array perpendicular to the array direction of the localization of the problem of low resolution. [PERSON] et al. [25] applied the beam forming technique to robotic sound source localization and used the minimum-variance distortion-free response adaptive beam forming algorithm for sound source localization, which provided a higher localization accuracy compared to the conventional beam forming algorithm. They proposed an improved cross-shaped microphone array data processing method to reduce the computational burden of the beam forming algorithm when used with the 2D microphone arrays. [PERSON] et al. [26] proposed a virtual primitive localization method based on planar array beam formation, which symmetrically split a uniform square array into four subarrays projected into virtual primitives through subarray beam formation and constituting a four-primitive cross array. The direction of incoming waves was calculated by combining the estimated wave directions by the four-primitive cross arrays.