Helmholtz was a physicist some years back who calculated that a "volume" with a hole in it would have a very precise frequency that could be calculated from just the volume of the container and the area of the hole (Or the sum of the areas of all the holes). He constructed a bunch of glass globes calibrated (according to formula) to use as standards for determining the pitch of tones he was studying.

Since the formula is pretty simple - and extremely accurate - it's become customary to refer to the main "resonant frequency" of a bucket - or other more or less bulky "noisemaker" as its "Helmholtz frequency." Things like guitar bodies can be described by their "Helmholtz" freqency, but of course they're complex enough that you have to also talk about plate frequencies and buzz tones and such. A "mostly closed" gutbucket can produce a fairly sharp "boom" at a specific pitch, if you work on it a bit.

Trivial example, the ocarina. It has a fixed internal volume, but the number of holes that you open determines the pitch. If the holes are all about the same size, it doesn't much matter which ones you open - just how many of them. It makes for a really simple fingering chart.

John