Mallet Hardness and Overtones - a visualization
The tool I used for this (Insight) is part of Izotope's mastering suite of software. This is not its normal use, but it provides a nice visualization of acoustic energy in real time across the frequency spectrum. Something went awry with the way that Quicktime did the screen capture, so by the end of the video, the waveforms are appearing significantly later and out of sync with the sound, but you will be able to still associate them and see the differing harmonic content. The frequency is represented along the bottom of the graph.
Hardware here was: a pair of Earthworks SR-25 mics (very flat response), Neve 5024 preamps, Universal Audio interface. There was no processing on the sound at all.