So that hole at the base of the handpan is not to be under estimated and it actually affects many desirable qualities for a great instrument !
- it make a great handle 😉
- it allows access for fine tuning and re-tuning
- it will determine the ding’s main note in 2 ways as in tuning the Ding to the same note as the Gu or alternativly tuning the Gu (hole) to have the 3rd (5th harmonic) of the Ding.
- and … It will determine how loud your sound will be.
So how to determine the proper size for the Gu opening ? Well as with anything it can be complicated but the short answer seems to be 1/4 of the total chamber volume. So fill your handpan with sand or water, then measure how much sand you got in there. 1/4 of that amount will tell you how much volume the close to ideal hole opening requires… you still need to convert that volume to a diameter so to make the proper hole 😉
This blog did a nice write up on it : http://www.apsimplepsaltery.com/appendices/helmholtz/
and here is a volume to diameter helper : https://www.calculatorsoup.com/calculators/geometry-solids/
So recently there is something going on about tuning the bottom shell GU hole to a Bb but i’m not clear on that yet … let try it !