MrTree has it right. I'd add the more research-y tools like CAT and NMR techniques and the Shigometer techniques I published on in the 1980-90s, When the machinery/electronics is working perfectly and is optimally positioned (no small thing), it will give you data from which may be inferred some aspects of wood density (including voids), or moisture content, or electrical resistance. All of those methods have their shortcomings...as does sounding with a mallet. Some of you may have greater practical experience than me with the instrumentation, but when I've collaborated with or stood over the shoulder of experts/proponents, accuracy and precision of the inferences ranged from "maybe OK" to "far out in left field".
I have to ask, even if the precision and accuracy was high, would it help make the on-ground-management decisions? Maybe in very high value specimen trees. Maybe not. Too me, variation in wood density or even electrical resistance (with which I have had the most experience) contains much less useful information than, say, the presence of cracks, sapwood rot, vigorous (or not) woundwood ribs for evaluating specimen trees.
When I used to give trainings to arborists on Shigometer technique, it was clear that practitioners wanted a box with just two lights: a green one for "good" and a red one for "bad". Instead, we gave folks instructions on how to first become familiar with patterns of mobile ions over many trees before trying to unravel the condition of individuals. Most folks just glazed over.