Speaker Box Calculator
Enter your driver's Thiele-Small parameters (Vas, Qts, Fs) to instantly calculate the ideal enclosure volume for a sealed or ported (bass reflex) box. The calculator also gives you the system tuning frequency, the -3 dB cutoff point, and the required port length for ported designs. Switch between metric (litres, cm) and imperial (ft3, inches) units as needed.
What is a speaker box calculator?
A speaker box calculator uses a subwoofer or woofer driver's Thiele-Small (T/S) parameters to determine the correct internal volume for its enclosure. The three key parameters are Vas (equivalent compliance volume), Qts (total quality factor), and Fs (free-air resonance frequency). These figures appear on every driver's spec sheet and fully describe how the speaker behaves mechanically and electrically. Plugging them into the appropriate formulas gives you the net internal air volume your box needs, how low it will play (-3 dB point), and, for ported boxes, the dimensions of the port tube that controls bass tuning.
Sealed vs. ported enclosures: which should you choose?
Sealed boxes are simpler to build, more forgiving of volume errors, and produce tight, accurate bass with a gentle 12 dB per octave rolloff below the cutoff frequency. They are preferred for music listening where transient accuracy matters. Ported (bass reflex) boxes extend bass output lower and play louder at the tuning frequency, but they use more space, require a precisely tuned port, and roll off steeply at 24 dB per octave below the port tuning frequency. Use the Efficiency Bandwidth Product (EBP = Fs / Qts) as a guide: below 50 favors sealed, above 100 favors ported, and between 50 and 100 either works. Drivers with a high Qts (above 0.7) almost always perform better in sealed boxes.
How to read and use Thiele-Small parameters
Vas is the volume of air whose spring stiffness equals the driver's suspension compliance - a 30-litre Vas means the driver needs an enclosure roughly in that range to work correctly. Qts combines the driver's electrical damping (Qes) and mechanical damping (Qms) into a single figure describing how the cone motion is controlled near resonance. A low Qts (under 0.4) means the driver is heavily damped and suits a ported box; a high Qts (over 0.7) needs the extra damping a sealed box provides. Fs is the free-air resonance: the lower it is, the deeper the bass the driver can reach. Published T/S parameters are measured under small-signal conditions; at high power levels they shift, so always verify critical designs with measurement software.
Port length and tuning frequency explained
In a ported box, the port (a tube or slot in the cabinet wall) acts as a Helmholtz resonator. Its resonant frequency - the tuning frequency Fb - creates a bass boost that extends the system's useful low-frequency output. Below Fb the driver loses back-pressure support and cone excursion rises rapidly, risking damage at high volume. The port length required for a given Fb depends on the port diameter and the box volume: a wider or longer port tunes lower, and more ports in parallel behave like one larger port. The formula used here is the standard acoustic approximation Lv = (23562.5 x d^2 x Np) / (Fb^2 x Vb) - 0.732 x d (all dimensions in centimetres, volume in litres). If the calculated length comes out negative, your port diameter is too large for the chosen tuning frequency and box size - reduce the port diameter or increase the number of ports.
Qtc target values for sealed boxes
| Qtc target | Alignment name | Characteristic | Best for |
|---|---|---|---|
| 0.5 | Overdamped | Tightest transients, highest F3 | Music accuracy, small rooms |
| 0.577 | B2 (Butterworth 2nd order) | Critically damped, clean rolloff | Home audio, music |
| 0.707 | B3 / Butterworth | Flat response, standard choice | General purpose (default) |
| 0.8-1.0 | Quasi-Butterworth | Slight bass lift, lower F3 | Home theater, movies |
| 1.0-1.2 | Chebychev | Noticeable bass hump, deeper bass | High SPL, car audio |
Common target system Q values and their acoustic characteristics for sealed enclosures.
Frequently asked questions
What are Thiele-Small parameters and where do I find them?
Thiele-Small parameters are a set of electromechanical measurements that describe a speaker driver's behavior. The three you need for box design are Vas (equivalent volume, in litres or cubic feet), Qts (total Q factor, dimensionless), and Fs (free-air resonance, in Hz). They appear on the driver's spec sheet or data sheet, usually on the manufacturer's website or in the product listing. Some sellers measure their own T/S parameters, which can differ from published values, so measuring your specific driver with software like REW or DATS gives the most accurate box design.
What does Qtc mean and what value should I target?
Qtc is the system quality factor of the driver inside its sealed box. It controls the shape of the bass rolloff. A Qtc of 0.707 is the Butterworth alignment - it gives the flattest possible frequency response and is the standard starting point. A Qtc below 0.7 (such as 0.5 or 0.577) gives tighter, more accurate bass that falls off earlier in frequency. A Qtc above 0.7 (such as 0.9 or 1.0) adds a slight bass lift near the cutoff and extends the low end, but at the cost of some booming quality. For music listening, 0.707 is the default. For home theater where deep bass impact matters, 0.9 to 1.0 is common.
How do I calculate the internal volume of a box from its dimensions?
Measure the internal dimensions (width, height, depth) in the same unit and multiply them together. If you are working from external dimensions, subtract twice the board thickness from each dimension first: internal width = external width - 2 x board thickness (typically 18-19 mm for 3/4 inch MDF). Then subtract the driver displacement (a 12-inch subwoofer displaces roughly 2-3 litres) and any port volume (port tube cross-section area multiplied by its length). The result is the net air volume the driver sees, which is what the Thiele-Small formulas require.
Why might my calculated port length be negative?
A negative port length means the port diameter you entered is too large relative to the box volume and target tuning frequency. The port needs a certain mass of air (proportional to its length) to resonate at the target frequency, and a very wide port achieves the same mass in an impossibly short - or even zero-length - tube. Fix it by reducing the port diameter, using more ports (which each can be narrower), increasing the box volume, or accepting a higher tuning frequency.
How much does wood thickness affect the internal volume?
Significantly for small boxes. A typical 18 mm (3/4 inch) MDF panel removes 36 mm from each internal dimension. For a 12-inch cube-shaped box planned at 40 cm external, the internal side is 40 - 2 x 1.8 = 36.4 cm, giving an internal volume of 36.4^3 = about 48 litres instead of 64 litres for the external size. Always calculate from internal dimensions, not external.
Can I use this calculator for full-range speakers and tweeters?
The Thiele-Small sealed and ported formulas apply to woofers and subwoofers. Full-range drivers can use the same approach, but very small satellite speakers often work best in small sealed boxes designed by ear or with simulation software. Tweeters do not require a calculated enclosure - they are usually mounted in an open rear chamber or a sealed back cup. This calculator is most useful for drivers with an Fs below 150 Hz, which covers most woofers, midwoofers, and subwoofers.