An impact echo (IE) test is a common, low-strain integrity test used to evaluate the thickness and inner condition of concrete plates. The vibration signal propagating along the thickness of a plate is of concern. However, the measured signal includes other waves, such as surface waves, reflective waves, noises from the environment, and so on. These unwanted signals affect the detection of plate integrity. The Hilbert–Huang transform (HHT), which is based on complementary ensemble empirical mode decomposition (CEEMD), has been found to distinguish and interpret IE signals effectively. This method breaks signals down into several meaningful ingredients called intrinsic mode functions (IMFs). CEEMD can also substantially reduce mode mixing and the final residue. The critical parameters of CEEMD are studied by coherence in the frequency domain, and their performance is demonstrated in this study. A Hilbert spectrum analysis is also proposed to distinguish various types of IE signals and enhanced to explain its properties. After obtaining the IMF of the echo wave, the corresponding frequency of the peak magnitude could be used to calculate the interface position. In addition, an approach is also proposed to calculate the depth interface using the IMF phase information, and the result shows that it can be evaluated successfully.
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