The expence of UT
This has ben a very enlighteing topic. My experience with UT is that it can penetrate many layers and when connected to a acoustic recorder can map irregularities in solid structures, ie testing for cracks and failures in composites as well as metallic structures. We used an induction point UT unit and followed with the transducer microphone to check early Boeing wing attachment irregularities at their fusilages and addressed rivet anomalies in those structures. The point is if you can introduce an acoustic signal with a resonance, one you can barely, hear but feel, it can penetrate with enough energy to bounce around in the structure with energy and be picked up on a tiny microphone. A electric toothbrush of the ultra sonic variety will induce a fixed gain signal, enough energy to penetrate, though not as variable as one of the expensive lab grade units, but it is 1/10th the cost. Would like to see a side by side comparison on this hack comparing the $1200.0 - $5000. dollar packages to a $100. Sonicare toothbrush.