Why box volume and port length matter
A subwoofer driver does not work alone: the air inside its enclosure acts like a spring behind the cone. In a sealed box, that spring controls the bass. In a ported, or bass-reflex, box, the air in the port and the air in the box form a resonator that reinforces the lowest notes around the tuning frequency. Getting the volume and port length right is what turns a driver into a good subwoofer.
Net internal volume
The calculator starts from the outside dimensions:
- inside dimensions = outside dimensions − 2 × wood thickness;
- gross volume = inside width × inside height × inside depth;
- net volume = gross volume − driver displacement − port volume.
Driver displacement is listed on the spec sheet, typically 0.05 to 0.2 cubic feet for a 10 to 15-inch driver. Bracing also takes a little space; subtract it if it is large.
Port length
A ported box is a Helmholtz resonator, like the note you hear when blowing across a bottle. Its tuning frequency depends on the box volume and the port’s area and length:
Effective length = c² × A ÷ (4π² × Fb² × V)
where c is the speed of sound, about 343 m/s, A the total port area, Fb the tuning frequency and V the net volume. Because air just outside each end of the port also moves, the physical port is shorter than the effective length, by about 0.73 × the diameter for a port with one flanged end. As the port takes up volume itself, the calculator repeats the calculation until it settles.
Example
A box of 24 × 15 × 16 inches outside, in ¾-inch MDF, has about 2.55 cubic feet inside. With a driver displacing 0.1 cubic foot and a single 4-inch port, tuning to 32 Hz needs a port about 10.9 inches long, leaving about 2.37 cubic feet net.
Choosing a port size
A small port is easy to fit but makes noise at high volume as air rushes through it. Many builders aim for at least 12 to 16 square inches of port area per cubic foot of box for subwoofers. Larger ports need to be longer for the same tuning, so slot ports or elbows are often used.
Limits
This calculator gives the tuning of a box; it does not tell you whether that box suits your driver. Use the driver’s Thiele-Small parameters (Fs, Qts, Vas) and a modeling program to choose the volume and tuning. Very short or very long ports, and ports too close to a wall, can shift the real tuning.