Abstract: Effective signal processing technology should be used to improve signal-to-noise ratio in air-coupled ultrasonic testing, because of feeble signal and low signal-to-noise ratio. Pulse compression with non-linear frequency modulation is applied in air-coupled ultrasonic testing, based on the research of pulse compression with linear frequency modulation, some improved method are presented. Principle of non-linear frequency modulation pulse compression is introduced. The effect of frequency characteristic of transducers is analyzed, because of the particularity of ultrasonic testing process. The affecting of stimulating mode to ultrasonic signal and pulse compression to signal-to-noise ratio are researched by comparing experiment results. The imaging testing experiment of fiberglass composite sample is accomplished with air-coupled ultrasonic C-scan testing system. Experiment results indicated that the pulse compression with non-linear frequency modulation is effective to improve the effect of pulse compression, and enhance the effect of air-coupled ultrasonic testing.

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