Helmholtz Resonator Calculator
Calculate the resonant frequency of vented enclosures and ports. Adjusts for air temperature and end correction factors, essential for acoustic engineering and speaker design.
About
A Helmholtz resonator describes the phenomenon of air resonance in a cavity with an opening, such as a bass reflex speaker port or the sound created by blowing over a bottle. Accurate calculation of this frequency is critical in acoustic engineering to tune enclosures to specific musical notes or to design architectural absorbers that eliminate unwanted room modes.
Standard formulas often fail in real-world application because they ignore environmental variables. This calculator accounts for the Speed of Sound (c) as a function of temperature, and applies the "End Correction" factor (k). The air inside the neck of the port effectively has a mass slightly larger than the physical dimensions of the tube due to air entrainment at the opening. Ignoring this results in a tuning frequency higher than measured reality.
Formulas
The resonant frequency fH is determined by:
Where effective length Leff includes the end correction:
Speed of sound c depends on Temperature (T in °C):
Reference Data
| Port Type | End Correction (k) | Usage Context |
|---|---|---|
| Flanged End | 0.85 | Port ends flush with a large flat wall (baffle). |
| Free Air (Unflanged) | 0.614 | Port end sticks out into open air (pipe end). |
| One Flanged, One Free | 0.732 | Typical Bass Reflex (flush outside, floating inside). |
| Slot Port | Complex | Rectangular vents (uses hydraulic diameter). |