Research on two inverse methods for characterization of an inaccessible thin film under a plate by low-frequency ultrasound

Two ultrasonic methods for determining the parameters of a composite consisting of a soft thin film attached to a hard plate substrate were studied,by resonance spectra in low frequency region,using its resonant frequencies and amplitude spectra,respectively.An analysis of the sensitivity function,the objective function and the error transfer function were presented for discussing the effect of the two methods for both one parameter and two-parameter inversion. In experimental study,the two techniques were employed to characterize a composite consisting of a film attached beneath to an Al plate substrate with a broadband 7 MHz center frequency transducer.It was observed that the two methods' results were similar for single parameter inversion and the method based on resonant frequencies got better result for two-parameter inversion under the same experimental condition.The relative errors of the method based on resonant frequencies for inversing acoustic impedance and time-of-flight were 3.4% and 4.7%,respectively,and the method based on amplitude spectra got no results.