From

http://www.askmar.com/askmar/Noise/Noise%20FAQ/FAQ_Text.html

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6.5 Why does blowing over a bottle make a note?

Resonance in acoustics occurs when some mass-spring combination
is supplied with energy. Many musical instruments rely on air
resonance to improve their sonority. If you blow across the
mouth of a bottle you can often get a note. The bottle behaves
as a Helmholtz resonator. The main volume of air inside the
bottle is analogous to a spring, whilst the "plug" of air in
the neck acts as an attached mass. The resonant frequency is
roughly given by:

     f = { c sqrt (S/LV) } / 2pi

c is velocity of sound
S is the surface area of the neck opening
V is bottle volume
L is the effective length of the neck i.e. the actual length
plus ends correction. Ends correction ~ 1.5 times radius of
neck opening

Example: A 75 cl (7.5E-4 m^3, approx. a "fifth") sized wine
bottle with neck diameter 19 mm, bottle neck length 8 cm,
air temp = 20 degrees C. The calculated resonant frequency
is 109Hz, actual resonance was 105Hz.

Helmholtz resonators are sometimes employed as a means of
passive noise control in air conditioning ducts. They may
also be hidden in the wall design of auditoria and offices
in order to improve the acoustics.

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Matthew Lamkin wrote:
>
> Hi there guys, can anyone tell me where I can find the equations
> (preferably the shorter version) for working out a Helmholtz resonator?
> (on the w.w.web that is).
>
> We can seem to find it in any physics books that we have.
> Tia - Matt.