Abstract：The acoustic holography technique has high spatial resolution and fast calculation speed, and it can visualize structural waves propagating inside a structure.This technique provides a significant direction for the super-diffraction limit resolution detection.Here, the propagation of near field ultrasonic wave in thin workpieces and the law of interaction between acoustic field and micro defect were studied.The quantitative relations among longitudinal wave signal, workpiece structure and defect types were established based on the near field acoustic holography (NAH).A method to judge if there are defects in near field area was found with the acoustic field characteristics of workpiece surface.Finally, the calculation results were compared with those of the finite element simulation to verify the correctness of the method.It was shown that the method provides a certain guide for improving the near field detection resolution and the feature extraction of micro defects.
