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The heat treatment of the Caribbean steel drum has been found to involve strain ageing and is especially prominent in drum steels containing 0.03–0.04 wt% C. The optimum strain-ageing conditions appear to be about 350 °C for 10 min, and either water quenching or air cooling produce similar ageing effects (hardness increases) ranging from about 5 to 20%. The strain ageing combined with the strain hardening applied to the drum-head sinking and note-fabrication processes, produces a requisite elastic–plastic interaction, which allows for multiharmonic tuning and the creation of the unique chromatic tones and harmonic overtones that are a characteristic of the various instruments. These unique features of note vibrations were illustrated by comparing dynamic impact hardness profiles with corresponding, static Vickers hardness measurements for actual, tuned notes and the same, corresponding notes extracted from the drum head, respectively. Elastic–plastic and plastic-hardness profiles were compared in unique colour maps. Microstructural analyses by light metallography and transmission electron microscopy illustrate corresponding dislocation substructures and carbide precipitation. Finally, the analysis and comparison of acoustic spectra for specific steel-drum note zones illustrates their complex, non-linear behaviour, and the role that deformation-induced defects play in acoustic dispersion and multiharmonic signal production.

Steel-drum fabrication, especially the sinking of the drum head (also referred to as the “pan”) by hand with a hammer, has been examined in detail utilizing light metallography (LM) and transmission electron microscopy (TEM) to characterize residual microstructures corresponding to reductions in thickness of up to 50 % at the bottom of the drum head. Dislocation densities in the low-carbon (0.01–0.05 wt % C), ferritic steels can exceed 1010 cm-2. Simulations of simple, ideal, free circular notes utilizing 316 stainless-steel plates (0.05 wt % C), cold rolled to reductions up to 40%, revealed that deformation (per cent cold reduction) has an important effect on the acoustic spectrum, especially harmonic spectra. Harmonic-node splitting was observed for thin circular plates (0.076 cm thick); the frequency difference was 60 Hz at 20% cold reduction and 160 Hz at 40% cold reduction. These dispersion effects, due to deformation-induced microstructures, as well as irregularities in the note geometries and thicknesses, point to the complex and non-linear acoustic features that contribute to the unique sounds of the Caribbean steel drum.