Helmholtz resonance is the phenomenon of air resonance in a
cavity, such as when one blows across the top of an empty
bottle. The name comes from a device created in the 1850s by
Hermann von Helmholtz, the "Helmholtz resonator", which he, the
author of the classic study of acoustic science, used to
identify the various frequencies or musical pitches present in
music and other complex sounds. Qualitative explanation  When
air is forced into a cavity, the pressure inside increases. When
the external force pushing the air into the cavity is removed,
the higher-pressure air inside will flow out. The cavity will be
left at a pressure slightly lower than the outside, causing air
to be drawn back in. This process repeats with the magnitude of
the pressure changes decreasing each time. The air in the port
(the neck of the chamber) has mass. Since it is in motion, it
possesses some momentum. A longer port would make for a larger
mass, and vice-versa. The diameter of the port is related to the
mass of air and the volum