difference between acoustic emission testing and ultrasonic

huangyhg

difference between acoustic emission testing and ultrasonic
please help
   can anyone tell me the difference between testing with  an acoustic emission tester and an ultrasonic scanner test. i have use a ultrasonic scanner, but never aacoustic emission tester.  
more information on what you are testing and why please.
generally, ae is passive (receiver), ultrasonic is active (transmitter and receiver).
daveminter,
    thanks for you answer. just what i needed.
                                jay