6.5 Why does blowing over a bottle make a note ?
Description
This article is from the Acoustics FAQ, by Andrew Silverman with numerous contributions by others.
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 soundS is the surface area of the neck opening
V is bottle volume
L is the effective length of the neck ie the actual length plus ends correction. Ends correction ~ 1.5 times radius of neck opening
Example: A 75 cl (7.5E-4 m^3) wine bottle with neck diameter 19 mm, bottle neck length 8 cm, air temp = 20 degC calculated resonance = 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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