Abstract: To increase the signal and the signal-to-noise ratio in air-coupled ultrasonic testing, a study of application of linear-frequency-modulation based pulse compression technology in air-coupled ultrasonic testing is carried out. The fundamentals of linear-frequency-modulation based pulse compression technology are introduced. The design and implementation procedure of matched filter is described. The relationship between parameters of pulse compression technology and the test results is analyzed. Experimental researches on the influences of different excitation modes on the received signal of air-coupled ultrasonic transducer and the actual effect of pulse compression technology on the improvement of signal-to-noise ratio are carried out. An air-coupled ultrasonic C-scan testing system is set up with pulse compression technology and test experiments are done with carbon fiber reinforced plastics(CFRP) plates. Experimental results show that the transducer’s energy conversion efficiency with continuous wave excitation is better than that with pulse wave excitation, especially when the frequency characteristic of excitation signal is same as that of the transducer. The signal-to-noise ratio of air-coupled ultrasonic testing is enhanced effectively with pulse compression technology, and the validity of pulse compression technology is testified by ultrasonic imaging results.

Key words: Air-coupled ultrasonic testing, C-scan imaging, Linear frequency modulation, Pulse compression technology