Studies Of The Structure Impacts On Acoustic Transmission Properties

As an important carrier of information, acoustic wave plays a significant role in human life and production. Acoustic wave can present different properties in different structures, which can be used in detection. This thesis establishes two-dimensional aluminum plate models based on finite difference time domain method(FDTD), and studies the structure impacts on acoustic transmission properties for two cases of varying sample sizes and fixed sample size via numerical simulations.For the case of changing sample size: when the aluminum plate contains one structure filled with air, the transmitted acoustic power significantly increases when the plate width is below 8 wavelengths of the acoustic wave, and its largest relative change compared to the mean power for plate width above 20 mm can reach 10.5 times. This implies strong size effect exists. When we change the number and material of the structures, size effect will differ. The results show that when the sample size is below some critical value, size effect must be considered in ultrasonic testing. However, size effect can be neglected when the sample size is above this critical value. Therefore, the sample size effect should be seriously taken into account when conducting ultrasonic testing for small sized samples especially on the width of a few wavelengths to reduce errors.For the case of fixed sample size: the change of structure position and size will influence the transmission properties of the acoustic wave greatly. The power of the transmitted acoustic wave is very large when the structure is positioned around the center of the plate. The impact of the structure position on its acoustic transmission properties is very small when the diameter of the structure is half the wavelength of the acoustic wave.The change in the acoustic frequency will also have an impact on acoustic transmission properties. It is demonstrated that the difference in structures can result in the screening of acoustic wave at special frequency, which means the acoustic wave at proper frequency should be chosen to obtain good signal in detection. Also, the design of different structures is expected to be applied as acoustic filters. As for the square arrays of sub-wavelengthstructures, the inter-hole distance can affect the transmitted acoustic spectral feature.Results show that the change of the arrays and inter-hole distance can influence the peak of the spectrum. The root mean square deviation(RMSD) of the transmitted wave spectrum under different inter-hole distance compared to the result of a plate without structures were extracted as the feature index. It is shown that RMSD changes for different arrays and inter-hole distances. These results can lay a good foundation for detecting multiple sub-wavelength defects in ultrasonic testing.