# A Review of Acoustic Emission Source Localization Techniques in Different Dimensions
## 3 Sound Source Localization Methods for Two-Dimensional (2D) Structures
### Strain Rosette Technique  
Another classical method for acoustic source localization is the rosette method, in which the transducer arrays are arranged in the shape of a flower and the intersection of the propagation directions determined by two rosette arrays gives the acoustic source location when the transducer array direction is in the same direction as the main strain.  
The advantage of this technology is that it is not affected by noise and does not require the arrival time of waves. The limitation is that this technology is not applicable  
Figure 6: (**a**) Beam forming localization method [22]. (**b**) Beam forming localization method using L-shaped sensor array.  
to anisotropic plates, as it requires the principal strain direction to be consistent with the group velocity (or energy velocity) direction.  
[PERSON] et al. [28] proposed a method based on piezoelectric transducer rosette junctions for passive damage or impact localization in anisotropic or geometrically complex structures. A rosette junction consists of rectangular macro-fiber composite (MFC) transducers exhibiting a highly directional response to ultrasonic guided waves. [PERSON] et al. [29] proposed a method using MFC transducer arrays arranged in rosettes for fast impact localization in isotropic and composite aircraft panels. Each rosette utilized the directional response of three MFC transducers to determine the direction of the incident wave generated by the shock source as the principal strain angle. Two rosettes sufficed to determine the impact location from the intersection of incoming wave directions.  
[PERSON] et al. [30] achieved sound source localization in plates by arranging the sensors in a rosette array. This method requires the propagation direction to coincide with the main strain direction without knowing the wave velocity (Figure 7a). [PERSON] et al. [31; 32; 33] measured the wave stress differences at six piezoelectric sensors, as shown in Figure 7b, used continuous wavelet transform to determine the bending Lamb wave arrival times, and then solved the nonlinear equations to obtain the coordinates of damage location and bending wave velocity by combining backtracking techniques related to global line search and local Newton iterative methods. [PERSON] et al. [34] achieved damage localization on carbon-fiber-reinforced composite panels by combining arrival times and wave mode analysis from the data acquired by three closely spaced sensors, as shown in Figure 7c. [PERSON] et al. [35] investigated the effect of various rosette configurations, including 45\({}^{\circ}\) rectangular, 135\({}^{\circ}\) rectangular, 60\({}^{\circ}\) delta and 120\({}^{\circ}\) delta, on the accuracy of wave propagation direction estimation. The results showed that the 135\({}^{\circ}\) triangle configuration provided the superior results and that the errors depended on the size and position of each sensor in the rosette.  
Figure 7: (**a**) Rosette array localization method. (**b**) Localization method using six sensors. (**c**) Three closely placed AE sensors for AE source localization.  
[PERSON] et al. [36; 37] proposed an acoustic source localization utilizing transducers arranged in an isosceles triangle, which utilized the arrival time difference. The method was not only applicable to damage detection in isotropic materials but also to anisotropic structures. The method's performance has been continuously improved [38; 39; 40] (Figure 8b).  
[PERSON] et al. [41] introduced a localization method for an AE source that produced a rhombic or elliptical wavefront surface, as shown in Figure 8a.