Difference between acoustic emissions and ultrasonic waves
The propagation of a mechanical wave within a medium depends on the characteristics of that medium (e.g. density). 

An easy way to experience this is to knock a piece of material with a sharp hit and listen to it trying to dissipate the energy you gave it. Consider for example a bell. When you hit it, it goes bonnnnnnnnnnng at some tone. If you were to weld a few rings around the bell and try to hit it again, it would go bonng. Different density / stifness of the material, different sound.

Acoustic emissions are akin to the sound of the bell and the cause of the "ringing" are the internal cracks. Consider for example the sound of ice cracking (similar sounds are produced even as ice cracks on its own, without forcing it externally) or even closer to your question, glass cracking under thermal shock.

Within a block of ice, the acoustic emissions are low enough in frequency that can be heard. But, if that was a piece of kevlar breaking, the disturbance would propagate with different characteristics through the material. You can see that we are already at hundreds of kHz in this case and we would be needing special equipment to sense the material vibrating.

Hope this helps.