**4. Conclusions**

This paper presents an improved approach that combines the EEMD and 2D-MUSIC algorithm for the real-time impact localization on composite structures. The fast Hilbert Huang transform with an optimized EEMD algorithm is introduced to extract IMFs from impact signals in the whole frequency domain. Then, all IMFs in the whole frequency domain are directly used as the input vector of the 2D-MUSIC model separately to locate the impact source. The main advantage of the proposed method is its computational efficiency, which requires less time in comparison to the previous MUSIC-based impact location method, in order to achieve the same location accuracy. From experimental results on a cross-ply glass fiber reinforced composite plate, the wave fronts of impact signals can be easily found in the IMF4 or IMF5, and the corresponding spatial spectrum peak also easily located the impact source. Ten near-field impacts are in good agreement with the actual impacts. The maximum direction and distance error are 3◦ and 2 cm, respectively. The experiment on a cross-ply glass fiber reinforced composite plate proved that the use of the 2D-MUSIC algorithm improved by EEMD is a suitable approach for the real-time impact localization of composite structures.

**Author Contributions:** Y.Z. and J.X. developed the method. Y.Z. and X.C. conceived of, designed the experiments. Y.J. and J.P. performed the experiments. Y.Z. wrote the paper.

**Funding:** This work is supported by the National Natural Science Foundation of China (No. 51505339, No. 51575400), the Zhejiang Provincial Natural Science Foundation of China (No. LQ16E050005), the Postdoctoral Foundation of Zhejiang Province, China.

**Acknowledgments:** The authors would like to acknowledge the Natural Science Foundation of China, the Zhejiang Provincial Natural Science Foundation of China, and the Postdoctoral Foundation of Zhejiang Province, China.

**Conflicts of Interest:** The authors declare no conflicts of interest.
