I guess I must have gotten blown away by some of the hot air mentioned above, so I missed Gutbucketeer's question about a "tuned port."

Helmholtz did the calculations about a hundred years ago.

If you divide the VOLUME (cubic inches) of an enclosed air space by the "sum of the areas of all the holes" (square inches) in the container, the result is inversely proportional to the frequency at which the air will resonate. (Proportional to the wavelength of the sound.) More holes, or one bigger one, will raise the pitch. Of course there's a "scale factor" to multiply by that depends on whether you want cycles per second or radians per second of the sine wave, but you'll probably just "do it by ear" so it doesn't matter much what the number is.

If you start with a closed can, and put a fairly small hole in it, it will have an "air pitch." Make the hole bigger to raise the pitch to where you want it. A bigger can starts with a lower pitch, so if you want "bass" there's a sort of minimum volume needed for the can you start with, and if the hole gets too big relative to the volume the pitch gets "sloppy" - hence different sized instruments for different pitch ranges.

The air that goes in and out of the "can" doesn't really do all that much for the volume of a stringed instrument. It's the air pushed around by the vibrations of a large surface of the instrument that gets the noise out, hence the attention paid to creating an efficient "sound board" in most such instruments.

Note that you can't use an open bottom tub to "let the sound out" in any practical way and still have a "tuned" instrument.

The "f-holes" in a fiddle are made that way because very precise tuning is wanted there (although every builder has a different idea of what they should be tuned to). It's easier to "tweak" a crooked hole by making it a little "differently crooked" than it is to change the size of a round hole accurately while still keeping it round. (Guitars can (usually) use a round hole 'cause "pitching" the box is less critical when you play lots of strings with a range of frequencies at the same time.)

John