This paper presents the Electromechanical Impedance Spectroscopy (EMIS) method employing a novel piezoelectric wafer active sensor (PWAS) with sub-resonators, which can generate additional resonant peaks to enhance the impedance signature. In order to develop an in-depth understanding of the mechanism behind the sub-resonator effects, an analytical investigation is conducted first. The theoretical solution for the impedance of the new sub-resonator PWAS transducer is derived. Furthermore, numerical simulations are carried out to demonstrate the effectiveness of the new transducer to create additional resonant peaks. Harmonic analysis of coupled field finite element (FEM) models is conducted. Material degradations are modeled by altering the material properties like density and elastic modulus. Comparative investigations are carried out with both conventional PWAS transducers and sub-resonator PWAS transducers. EMI damage indices based on the spectral amplitude and frequency variation features are used to quantify the material degradation and simultaneously prove the superiority of the sub-resonator PWAS over the conventional PWAS. Additionally, a high-damping dog-bone specimen is employed to conduct the creep experiment lasting for twenty-four hours with a recording interval of two hours. The impedance spectra are obtained by the Bode-100 impedance analyzer. The experimental results further demonstrate the improved sensitivity of the sub-resonator transducer, which is in good agreement with the theoretical and numerical findings. The paper finishes with summary, concluding remarks, and suggestions for future work.