How this calculator works
A compact air cavity behaves like a spring while the air in the neck behaves like a moving mass. End correction includes air that moves just outside the opening. The default total correction of 1.7 equivalent radii is an approximate two-end model; change it for your boundary conditions. For noncircular openings the equal-area radius is an approximation. The cavity must be small relative to wavelength and the neck must remain acoustically short.
Formula
f = c/(2 pi) sqrt[A/(V Leff)]; Leff = L + k sqrt(A/pi). Box V = whl; sphere V = 4 pi r³/3; cylinder V = pi r²h. Opening area uses the selected geometry.
Worked example
With c = 100 m/s, A = 0.01 m2, V = 0.01 m3 and an effective neck length of 0.1 m, the resonance is 50.3292 Hz. Set end correction to zero when entering that effective length.
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Frequently asked questions
Does this predict the loudness at resonance?
No. Losses, coupling and radiation resistance determine the response strength and bandwidth.
Why can neck length be impossible?
A target frequency may require an effective length shorter than the chosen end correction, giving a negative physical length.
Can I use an arbitrary opening?
Yes, by entering its area. Its equal-area radius is only a simple estimate for end correction.