Theater Calculators

TV mounting & DIY

Helmholtz Resonator DIY Frequency Calculator

A bottle is a Helmholtz resonator. Build a cavity, a neck, and you can notch a single nasty mode. Measure after you cut.

Inputs

Units
m³

Internal volume, not the outside of the box. 1 L = 0.001 m³; 1 ft³ = 0.0283 m³.

m
m

Results

Neck area
0.001963m²
Tune frequency
33.8Hz

The Result is indicative and educational. It does not constitute a design, a technical opinion or medical advice and does not replace calculations or verification by a person with the appropriate qualifications and authorisations. Before purchase, installation or use in practice – in particular in electrical installations – the Result should be verified by a qualified specialist. Details: Terms of Service, point 4.

Example products

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Calculation method

A = π(d/2)². L_eff = L + 0.8√A. f = (343 / 2π) √(A / (V L_eff)).

Exact formula

f = (c/2π) √(A/(V L_eff)), L_eff = L + 0.8√A. Limits: no lining, no leaking joints, one degree of freedom, 20 °C, end correction 0.8√A ≈ 1.42 r (one flanged and one free end; a hole in a thick panel is nearer 1.7 r and tunes a few per cent lower).

Frequently asked questions

Multiple necks?

N identical necks act in parallel: calculate one neck, then multiply the frequency by √N. Do not enter one big equivalent diameter, because each neck keeps its own end correction.

Fill the box?

Light fill lowers Q and pulls f down a little. Recalculate after you stuff, then measure.

Port on the face?

End correction assumes a flanged-ish opening. Flush ports are close enough to start.

Where does it go?

Where the target mode has high pressure: a room corner, or against one of the two walls that mode bounces between. Near the middle of the room, at a pressure node, it does almost nothing.

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