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How It Works
A ported (bass reflex) subwoofer enclosure uses a tuned port or vent to extend low-frequency response. The port length and diameter determine the tuning frequency of the enclosure. This calculator solves two key equations:
- Port Length (Lv) — given the port diameter, box volume, tuning frequency, number of ports, and end correction factor.
- Minimum Port Diameter (Dmin) — the smallest port diameter that avoids audible wind noise (chuffing), calculated from the driver displacement volume and tuning frequency.
The end correction factor (k) accounts for the acoustic mass of air at the port openings. Common values are 0.732 (one free end, one flanged), 0.614 (two free ends), and 0.850 (two flanged ends).
Example Problem
A subwoofer box has a volume of 50 liters and a 7.62 cm (3″) diameter port. The desired tuning frequency is 35 Hz with one port and an end correction factor of 0.732 (one free end, one flanged).
Lv = (23562.5 × 7.62² × 1) / (50 × 35²) − 0.732 × 7.62
Lv = 1,368,584.625 / 61,250 − 5.578 = 22.34 − 5.578 ≈ 16.77 cm
Frequently Asked Questions
What is the end correction factor (k)?
The end correction factor accounts for the extra acoustic mass of air at the port openings. A flanged end (flush-mounted to the baffle) has more acoustic loading than a free end. Use 0.732 for one flanged/one free, 0.614 for two free ends, or 0.850 for two flanged ends.
What happens if the port is too small?
An undersized port causes air to move at high velocity through the opening, producing audible turbulence noise known as “chuffing” or port noise. The minimum diameter calculation ensures the port area is large enough to avoid this at the driver's maximum excursion.
Can I use multiple ports instead of one large port?
Yes. Multiple smaller ports can replace one large port. The calculator accounts for this with the number of ports (Np) parameter. The total port area is distributed across all ports, and the formula adjusts the required length accordingly.
What is the relationship between port length and tuning frequency?
Longer ports produce lower tuning frequencies for a given diameter and box volume. The relationship is inverse-square: doubling the tuning frequency reduces the required port length by roughly a factor of four (before end correction).
Can I use rectangular ports instead of round ones?
Yes. For rectangular ports, calculate an equivalent diameter using the formula D = 2 × (width × height) / (width + height). This hydraulic diameter can then be used in the port length equation. Rectangular ports are commonly built into the enclosure walls.
Related Calculators
- Subwoofer Box Enclosure Comparison Calculator — compare sealed, ported, and bandpass enclosure designs.
- AC Circuit Calculator — inductive and capacitive reactance for crossover design.
- Passive Crossover Calculator — design crossover filters for multi-driver speaker systems.
- Sound Wave Calculator — calculate wavelength at the port tuning frequency.
- Frequency Converter — convert tuning frequency units for vent design.
Lindeburg, Michael R. 1992. Engineer In Training Reference Manual. Professional Publication, Inc. 8th Edition.