Subwoofer Box Calculator
Calculate sealed and ported subwoofer enclosure dimensions from driver specifications. Enter values for instant results with step-by-step formulas.
Formula
Vb = Vas / ((Qtc/Qts)^2 - 1) for sealed | Fb = Fs x 0.9 for ported
For sealed boxes, Vb is the optimal internal volume calculated from Vas (equivalent compliance volume), Qtc (target system Q, typically 0.707), and Qts (driver total Q). For ported boxes, the tuning frequency Fb is typically set to 85-95% of the driver free-air resonance Fs.
Worked Examples
Example 1: 12-inch Sealed Subwoofer Box
Problem: Design a sealed enclosure for a 12-inch driver with Vas = 80L, Qts = 0.45, and Fs = 28 Hz targeting Qtc = 0.707.
Solution: Sealed volume: Vb = 80 / ((0.707/0.45)^2 - 1) = 80 / (2.468 - 1) = 80 / 1.468 = 54.5 L (1.92 ft3)\nSealed Fc = 28 x sqrt(80/54.5 + 1) = 28 x 1.571 = 44.0 Hz\nF-3 (Butterworth) = ~44 Hz\nDimensions (golden ratio): 14.9 x 24.2 x 15.0 inches\nMaterial: 3/4-inch MDF\nAdd 5% for displacement = 57.2 L actual build volume
Result: 54.5 L sealed | Fc: 44 Hz | F3: ~44 Hz | 14.9 x 24.2 x 15.0 inches
Example 2: 10-inch Ported Subwoofer Box
Problem: Design a ported enclosure for a 10-inch driver with Vas = 45L, Qts = 0.35, and Fs = 32 Hz.
Solution: Ported volume: Vb = 45 x 1.5 = 67.5 L (2.38 ft3)\nTuning frequency: Fb = 32 x 0.9 = 28.8 Hz\nPort diameter: 10 x 0.3 x 25.4 = 76.2 mm (3 inches)\nPort area = pi x (76.2/2)^2 = 4,560 mm2\nPort length = (23562.5 x 4560) / (28.8^2 x 67.5 x 1000) - 0.825 x sqrt(4560)\n= 1.92 - 55.7 = adjusted for formula\nF-3 = 28.8 x 0.7 = 20.2 Hz
Result: 67.5 L ported | Fb: 28.8 Hz | F3: ~20 Hz | 3-inch diameter port
Frequently Asked Questions
What is the difference between a sealed and ported subwoofer enclosure?
A sealed (acoustic suspension) enclosure is an airtight box that traps the air behind the driver, using it as a pneumatic spring to control cone movement. Sealed boxes produce tight, accurate bass with a gentle 12 dB per octave rolloff below the cutoff frequency. They are generally smaller, simpler to build, and more forgiving of design errors. A ported (bass reflex) enclosure has a precisely tuned opening or tube that allows internal air pressure to radiate sound at the tuning frequency, effectively extending bass output. Ported boxes produce louder bass output at and above the tuning frequency but roll off steeply at 24 dB per octave below it. They require more precise construction because incorrect port dimensions significantly degrade performance.
What are Thiele-Small parameters and why are they important for box design?
Thiele-Small parameters are a set of electromechanical measurements that describe a loudspeaker driver performance characteristics, enabling engineers to predict how it will behave in different enclosures. The three most critical parameters are Fs (free air resonance frequency in Hz), Qts (total Q factor representing the damping characteristics), and Vas (equivalent air compliance volume in liters). Fs indicates the lowest frequency the driver naturally resonates at. Qts determines whether the driver suits a sealed box (Qts above 0.5), a ported box (Qts below 0.4), or either (Qts between 0.4 and 0.5). Vas represents the volume of air that has the same compliance as the driver suspension. These parameters are measured by the manufacturer and published in driver specification sheets.
What is the ideal internal volume for a sealed subwoofer box?
The ideal sealed box volume depends on the driver Vas and Qts parameters and the target system Q (Qtc). For a maximally flat (Butterworth) response, the target Qtc is 0.707, which produces no response peak before rolloff. The formula is Vb equals Vas divided by the quantity (Qtc/Qts) squared minus 1. A higher Qtc (0.8 to 1.0) produces a smaller box with a slight bass boost near the cutoff frequency, which can sound punchier for music. A lower Qtc (0.5 to 0.6) requires a larger box but provides a more gradual, extended rolloff. For a 12-inch driver with Vas of 80 liters and Qts of 0.45, the ideal sealed volume at Qtc 0.707 is approximately 32 liters or 1.13 cubic feet.
How do I calculate port dimensions for a ported subwoofer box?
Port dimensions involve calculating both the port diameter and length. The port diameter should be large enough to prevent port noise (chuffing) at high output levels, with a minimum diameter of about 30 percent of the driver diameter. For a 12-inch driver, the port should be at least 3 to 4 inches in diameter. Port length is calculated using the formula that relates tuning frequency, port area, and box volume. Shorter ports tune higher, while longer ports tune lower. A common tuning frequency is 85 to 95 percent of the driver Fs. If the calculated port length exceeds the box depth, use a flared or slot port instead of a round tube. Port air velocity should remain below 17 meters per second to avoid turbulence noise, which may require increasing the port diameter for high-power applications.
What materials should I use to build a subwoofer enclosure?
The standard material for subwoofer enclosures is 3/4-inch (19mm) MDF (medium-density fiberboard) due to its density, uniformity, and resistance to resonance. MDF weighs approximately 48 pounds per cubic foot, which provides excellent damping of panel vibrations. Baltic birch plywood at 3/4-inch thickness is an alternative that offers better screw-holding strength and moisture resistance, making it preferred for mobile and professional applications. Never use particle board, standard plywood, or solid wood, as these materials either lack sufficient density or have grain patterns that create resonant modes. All joints should be sealed with wood glue and reinforced with screws or brads. Internal bracing with additional MDF strips across the widest panels prevents flexing at high output levels, which would otherwise waste energy and cause audible distortion.
How does enclosure shape affect subwoofer performance?
While the internal volume is the primary design parameter, enclosure shape influences both structural integrity and standing wave behavior. A cube is the worst shape because it creates strong standing waves at a single frequency across all three dimensions. The golden ratio (1:1.618:2.618) proportions distribute internal standing waves across different frequencies, reducing their individual severity. Wedge-shaped and trapezoidal enclosures further break up standing waves and are popular in car audio applications where they fit against angled surfaces. The location of the driver and port on the baffle also matters: offsetting them from the center of the panel reduces the excitation of the fundamental panel mode. Internal damping material such as polyfill or fiberglass batting absorbs mid-frequency standing waves and can make a sealed box perform as if it were slightly larger.