A closed chamber with a small hole exposed to normal atmospheric
pressure will act as an acoustic resonator. The resonant frequency is lowered
by making the cavity volume larger and raised by making the hole larger.
The Helmholtz resonator formula is fr = .25 C D/Sqrt(Pi V (L+.75 D)).
fr is the resonant frequency in Hz.
C is the speed of sound in meters/sec. All other quantities use mm.
pi = 3.1415962
D = hole Diameter, L= hole length
V= cavity volume
The formula was found in a Star Micronics catalog.