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Air-coupled ultrasonic testing is a possible implementation method for online seal defect detection of flexible package. However, large acoustic impedance difference between air and solid material leads to difficulties in coupling ultrasonic energy into solid materials and results in a poor signal to noise ratio (SNR). To solve this problem, pulse compression technique based on Golay codes has been applied in air-coupled ultrasonic testing of flexible package seal defect. The effects of different Golay codes (4, 8 and 16 bits) and Golay codes with different bit length (1, 2, and 3 circles) are investigated to obtain a reasonable parameter selection. The results demonstrate that the Golay coded pulse compression technique can improve the SNR ratio of the received signal remarkably, and suppressed the side lobe of the compressed pulse effectively. An improvement in SNR of the seal region image up to 9.4 dB is achieved, compared with the non-modulated excitation method.

Application of Golay Coded Pulse Compression in Air-coupled Ultrasonic Testing of Flexible Package Seal Defect