22. The "sink"
There are at least three acoustic reasons for sinking the bottom of the drum: First, to remove the low tone in the bottom. Second, to make the metal thinner in the middle where the highest notes are to be put. And third, to make an overall curvature that is suitable for the notes of the different pan types.
The most obvious acoustic reason for sinking the bottom of the drum to a round basin is that the spherical shape has a very high resonance frequency related to its size. This means that any tones generated by vibrations in the sunk bottom will be well above the pitch of the notes.
The deeper the pan is sunk, the thinner the metal will be in the middle. Thinner metal is presumably beneficial for the tone generation of the higher notes, see the chapter about material. This may be one of the reasons why small notes in the outer ring often sound poorly and higher notes usually are put in the middle.
There also seems to be a certain relationship between the size of the notes and the needed overall curvature of the pan - small notes need a rounder arch of the sink while large notes need shallower sinking to get the right height. If a large note was made in a pan that is deeply sunk, its arch would presumably be too high to generate a good steel pan tone, see fig. 22.1.
Fig. 22.1 Relation between sink shape and note height.
The intricate relation between the shape of the sink and the note size is presumably due to the very special, cylindrical shape of the note dents. When elliptical notes are shaped in the sunk concave surface, the resulting note arch will be cylindrical with its largest curvature across the note. The length axis of the note is almost flat, a result that well coincides with the fact that most of the partials are generated from vibrations along the length axis of the note, see fig. 24.3 and the discussion of note arch further down.