Calculate a bass reflex port – length from volume and tuning

How it is calculated

The enclosure and the port form a Helmholtz resonator. Its resonance formula gives the effective port length; the end correction is subtracted from this.

L_{eff} = \frac{c^{2} \cdot A}{4 π^{2} \cdot f_{B}^{2} \cdot V_{B}} L = L_{eff} - k \cdot D

$A$ – total port area of all ports in m²
$f_{B}$ – tuning frequency in Hz, $V_{B}$ – net volume in m³
$c$ – speed of sound, here at 20 °C (343.4 m/s)
$D$ – inner diameter of one port, $k$ – end correction: 0.614 (both ends free), 0.732 (one end flush with the baffle), 0.850 (both ends flush)

Example

100 L net, tuning 40 Hz, one port with 100 mm inner diameter, one end flush: the effective length is approx. 14.7 cm, the port length to cut approx. 7.3 cm.

What to watch out for

Round ports only. Slot ports along an enclosure wall behave differently; this calculator does not handle them.
Do not make the port area too small. In ports that are too narrow, the air flows so fast at high levels that flow noise occurs. There is no universally valid minimum area – it depends on the cone area and excursion of the driver.
Subtract the port volume. The port itself takes up volume. With large ports, build the enclosure correspondingly larger.
Short ports only. The Helmholtz formula assumes that the enclosure and port are small compared with the wavelength. Very long ports should be simulated rather than calculated.

Plans take precedence

If a build plan specifies port dimensions, those apply. The calculator does not replace a simulation using the driver’s data.

Calculate a bass reflex port – length from volume and tuning

How it is calculated

Example

What to watch out for

Further reading

Sources