Hi Richard,
When "sounding" wood to find out whether it is sound or not just won't do. Search online for drills that measure resistance in wood. They are expensive but you can't beat them when wanting or needing empirical evidence.
When used they produce a graph with resistance in the y-axis and inches in the x-axis. It gives you a pretty clear picture of what is inside the faces. Small visual repercussions as well.
Hope this helps some. Perhaps you should find someone with one of these.
