Find net enclosure volume and port tuning for DIY speaker and subwoofer builds. Account for driver displacement, bracing, and panel thickness.
Need materials?
MDF board →
PVC pipe for port →
Woofer driver →
The internal volume of a box is not the same as the space you have for air. A typical mistake is calculating the inside dimensions by subtracting panel thickness from the outside, then calling that usable air volume. The truth is that the driver takes up space, the port takes up space, any internal bracing takes up space, and so does a crossover or amplifier if you put one inside. All of that counts against your net air volume.
Gross volume is the space inside the walls, measured from internal wall to internal wall. Net volume is what remains for air. It matters because the tuning frequency of a ported box is set by the air volume, not by the board dimensions. A 15-litre box with a 1.5-litre driver is really a 13.5-litre box for tuning purposes.
Driver displacement varies by size. A typical 6.5-inch woofer displaces roughly 0.3 to 0.5 litres. A 15-inch subwoofer might displace 2 litres or more. The motor structure, cone depth, and the rear dust cap all take up volume. If you know your driver's exact displacement, use it; the presets are estimates.
Port displacement is small but real. A 2-inch diameter port 6 inches long displaces about 0.15 litres. In a large home theater box this is noise, but in a compact car audio enclosure it can nudge the frequency down a few Hz.
Port length is calculated from three things: box volume, port diameter, and your target tuning frequency. A larger box or lower target frequency requires a longer port. The formula used is the Helmholtz resonator equation, which accounts for end correction (the effective acoustic lengthening of the air column at the port opening). This is typically 0.7 to 0.85 times the port radius, depending on whether the port is flanged or sits flush.
Port velocity is how fast the air moves in and out of the port. If it gets too fast (above 20 metres per second), the port creates audible chuffing noise. A larger port diameter lowers velocity for the same acoustic output.
These are typical displacements. Measure yours if you can.
| Size | Displacement (L) | Notes |
|---|---|---|
| 6.5 inch | 0.3-0.5 | Car audio, bookshelf speakers |
| 8 inch | 0.5-0.8 | Home theater, compact subwoofers |
| 10 inch | 0.8-1.2 | Mid-size subwoofers, car audio |
| 12 inch | 1.2-1.8 | Most common home subwoofers |
| 15 inch | 1.8-2.5 | Large subwoofers, PA cabinets |
| 18 inch | 2.5-3.5 | High-SPL subwoofers, PA |
Displacement includes the cone, the surround, the motor structure, and the rear magnet/pole piece. Measure by water displacement if precision matters, or use a speaker simulator tool with the driver's published specs.
| Use case | Frequency (Hz) | Notes |
|---|---|---|
| Home theater subwoofer | 18-25 | Deep bass extension. Requires larger box. |
| Music listening (flat response) | 25-35 | Depends on room. Higher frequency = smaller box. |
| Car audio, sealed box | 50-80 | Sealed boxes tune higher than ported. |
| Car audio, ported box | 32-45 | Loud at tuning frequency. Narrower bandwidth. |
| Compact/satellite speaker | 80-150 | Very small box or vented port. |
Tuning frequency is a choice you make based on the room, the subwoofer size, and your goal. Lower frequencies need larger boxes. Higher tuning frequencies work in smaller enclosures but restrict bass extension.
A sealed box means the enclosure is airtight. The air inside acts like a spring. As the woofer moves in, it compresses the air, creating a restoring force. Sealed boxes are easy to design, stable, and flat across a wide frequency range. They require more power because the air spring opposes the driver. Tuning does not apply to sealed boxes. Instead, the Thiele-Small parameters (Qts, Fs, Vas) of the driver define the system response.
A ported or vented box has one or more ports. The port creates a Helmholtz resonator with the box. At the tuning frequency, the port releases acoustic energy stored in the driver's motion, reinforcing output at that frequency. Ported boxes are louder at and just below tuning frequency and require less amplifier power. However, they have narrower bandwidth (response falls off quickly above tuning) and are less forgiving of design errors.
The trade-off is simple: sealed = flat and controlled but needs more power; ported = louder at one frequency but needs careful design. Both require an enclosure sized for the driver and the tuning goal.
Net volume is the actual amount of air remaining in the box after subtracting the volume lost to the speaker driver, the port, internal bracing, and internal crossover or amplifier components. It is different from gross internal volume. Most DIY builders calculate the internal dimensions by subtracting panel thickness from external dimensions, but then forget to subtract what the driver, bracing, and port themselves displace.
Port length is calculated using the Helmholtz resonator formula. The length depends on three things: the box volume, the port diameter (or area), and the target tuning frequency you want. A larger box or lower tuning frequency requires a longer port. A larger diameter port shortens the required length. The formula accounts for end correction, which subtracts an effective length equivalent to roughly 0.7 to 0.85 times the port diameter because the moving air column extends beyond the physical opening.
Yes. A driver typically displaces 0.3 to 2 liters depending on its size and magnet depth. In a small box, ignoring this can throw your tuning frequency off by several Hz. For example, a 15-litre box with a 1.5-litre driver is really an 13.5-litre box; that is a 10 percent error. Larger boxes tolerate it better, but accurate tuning starts with net volume.
End correction accounts for the fact that the acoustic length of the port is longer than its physical length. Air moving in and out of the port extends acoustically beyond the physical opening. The end correction factor is typically 0.7 to 0.85 times the port radius, depending on whether the port opening is flanged (reinforced at the edge) or free (open to air). This is why the calculator subtracts this effective length from the final port length calculation.
A Helmholtz resonator is an enclosed air chamber with a small opening. The air mass inside the opening and the air spring inside the box oscillate together at a specific frequency, called the resonant frequency. A speaker port creates a Helmholtz resonator with the box. At the tuning frequency, the port reinforces the woofer's output, extending bass response. This is why ported enclosures can play louder at low frequencies than sealed boxes of the same volume.
A sealed enclosure is airtight. The air inside acts as a spring, storing energy as the woofer moves. Sealed boxes are easier to design and always stable, but they require more amplifier power to reach the same level as a ported box. A ported (or vented) enclosure has a port, which is a tuned opening. At the tuning frequency, the port releases energy, lowering the amplifier load and extending bass response. Ported boxes are louder at and below the tuning frequency but less controlled above it.