Speaker Box Calculator
Calculate recommended enclosure volume for sealed or ported speaker boxes using Thiele-Small parameters.
This tool is for informational and educational purposes only. It is not a substitute for professional financial, medical, legal, or engineering advice. See Terms of Service.
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This calculator uses Thiele-Small parameters from your speaker's data sheet to recommend an enclosure volume. Here is how to use it:
- Find your speaker's Thiele-Small parameters. Look up Vas (equivalent acoustic volume in liters) and Qts (total Q factor) in the manufacturer's spec sheet or data sheet. These parameters describe how your driver behaves acoustically.
- Choose sealed or ported. Sealed enclosures are simpler to build, more forgiving of volume errors, and have tighter transient response. Ported (bass reflex) enclosures extend bass output and efficiency but are more sensitive to box tuning.
- Enter Fs for port tuning. If you select ported and enter the free-air resonance frequency (Fs), the calculator also shows a suggested port diameter and length in centimeters.
- Build to the calculated volume. Use the cubic feet or liter values to design the internal box dimensions. Subtract wood thickness from the external dimensions to get internal volume.
About Speaker Enclosure Design
The Thiele-Small parameters are a set of electromechanical measurements that describe a speaker driver's behavior in free air. Vas is the equivalent volume of air that has the same acoustic compliance as the driver's suspension. Qts combines electrical damping (Qes) and mechanical damping (Qms) into a single number that describes how peaked or flat the driver's response is near resonance.
For a sealed enclosure, the simplified recommendation is a box volume equal to 0.7 times Vas. This results in a moderate Q alignment that gives relatively flat bass response. For ported enclosures, the formula 15 * Vas * Qts^3.3 is a common empirical approximation. Drivers with low Qts (below 0.3) are generally better suited to ported or transmission line enclosures. Drivers with high Qts (above 0.7) work best in sealed boxes.
Frequently Asked Questions
Where do I find Thiele-Small parameters for my speaker?
Thiele-Small parameters are listed in the speaker's data sheet from the manufacturer. Search for your speaker model number followed by "data sheet" or "T/S parameters." Brands like Dayton Audio, Peerless, ScanSpeak, and Eminence publish detailed spec sheets. For car audio subwoofers, JL Audio, Rockford Fosgate, and Kicker publish T/S parameters on their product pages. If you cannot find them, you can measure them with a test microphone and software like DATS or REW.
What is the difference between sealed and ported enclosures?
A sealed (acoustic suspension) enclosure traps a volume of air behind the driver, which adds spring-like stiffness to the suspension and raises the resonant frequency. The result is tight, controlled bass that rolls off smoothly at 12dB per octave below the tuning point. A ported enclosure adds a tuned tube or slot that allows the rear wave of the driver to reinforce the front wave at a specific frequency. This extends bass output and improves efficiency, but the bass cuts off more steeply (24dB per octave) below the port tuning frequency.
How accurate are these enclosure volume recommendations?
The formulas here are simplified approximations good for a starting point. For serious builds, use acoustic simulation software like WinISD, BassBox Pro, or the free Unibox tool, which model the full frequency response curve and let you optimize for your specific goals. Real-world factors like internal bracing, driver magnet displacement, port wall thickness, and acoustic damping material all reduce the effective internal volume and should be accounted for by building slightly larger than the calculated target.
What happens if my box is too big or too small?
For sealed enclosures, a smaller box raises the system resonance and tightens the bass but reduces output at the lowest frequencies. A larger box lowers resonance and extends deep bass but can make the response peaked or boomy. For ported enclosures, box size and port tuning interact: too small a box with the same port will tune higher and lose deep extension. Too large and the port tuning drops too low, causing the driver to unload (lose control) at very low frequencies, which can damage the driver at high power. Aim within 20% of the recommended volume as a starting point.