What is the difference between acoustic emission testing (AET) and ultrasonic testing (UT) or other NDT methods?

There are two main differences between acoustic emission testing (AET) and ultrasonic testing (UT). First ultrasonic detector probes send ultrasonic waves and receive their echo waves in order to detect flaws or thickness. But acoustic emission waves are generated by the material or structures so the AE sensors only receive AE waves but not sending. UT is active testing but AET is passive detection. Secondly AET is a dynamic process and can detect and monitor active defects during the whole loading process while other NDT technologies can only deal with static state processes.