Disclosed herein is a non-destructive inspection (NDI) apparatus. The NDI comprises a main body and a modulated-frequency ultrasonic source, coupled to the main body. The modulated-frequency ultrasonic source comprises a low-frequency ultrasonic source, selectively operable to emit a low-frequency ultrasonic beam having a first frequency. The modulated-frequency ultrasonic source also comprises a high-frequency ultrasonic source, selectively operable to emit a high-frequency ultrasonic beam having a second frequency higher than the first frequency. The modulated-frequency ultrasonic source is configured to mix the low-frequency ultrasonic beam and the high-frequency ultrasonic beam at least at a surface of a structure to be inspected.