  The volume of air that goes in and out of a musical instrument's sound hole
is related to the sound hole's contribution to the volume of the sound.
Helmholtz's result for the simplest case of steady flow through an elliptical
hole is reviewed. Measurements on multiple holes in sound box geometries and
scales relevant to real musical instruments demonstrate the importance of a
variety of effects. Electric capacitance of single flat plates is a
mathematically identical problem, offering an alternate way to understand the
most important of those effects. The measurements also confirm and illuminate
aspects of Helmholtz's "bottle" resonator model as applied to musical
instrument sound boxes and sound holes.
