Among the various NDE techniques, acoustic emzsswn (AE) monitoring is arguably based on the simplest physical concepts, but is one of the most difficult techniques to practically implement. Nearly everyone has heard audible AE in the form of popping and cracking noises from materials under stress. Wood is an example of a material that produces such audible AE. As illustrated in the cartoon in Figure 6. 1 , a tree limb will emit cracking sounds when heavily stressed prior to breaking. One can estimate whether failure of the limb is imminent by listening to the amplitude and frequency of occurrence of these sounds.