Two ultrasonic methods were studied for determining the parameters of a composite consisting of a soft thin film attached beneath a hard plate substrate from resonant frequencies and amplitude spectra in low frequency region,respectively.Analyses of the sensitivity function, the objective function and the error transfer function were presented for discussing the effects of the two methods for both single parameter and two-parameter inversion.Two techniques were employed in experiment to characterize a composite consisting of a film attached beneath an aluminum plate substrate with a 7 MHz center frequency broadband transducer.It was observed that the results were similar for single parameter inversion and the method based on resonant frequencies got better result for two-parameter inversion under the same 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.
The super-cell plane wave expansion method is employed to calculate band structures for the design of a siliconbased one-dimensional phononic crystal plate with large absolute forbidden bands. In this method, a low impedance medium is introduced to replace the free stress boundary, which largely reduces the computational complexity. The dependence of band gaps on structural parameters is investigated in detail. To prove the validity of the super-cell plane wave expansion, the transmitted power spectra of the Lamb wave are calculated by using a finite element method. With the detailed computation, the band-gap of a one-dimensional plate can be designed as required with appropriate structural parameters, which provides a guide to the fabrication of a Lamb wave phononic crystal.
