Design custom sealed, ported, and bandpass subwoofer enclosures using your driver's Thiele-Small parameters. Enter Fs, Qts, Vas, and Xmax to get optimal box volume, port dimensions, and predicted frequency response — no downloads or sign-ups required.
The calculator supports isobaric (push-pull) configurations, passive radiator enclosures, and multi-driver setups. Advanced outputs include port air velocity, group delay, impedance curves, and thermal/mechanical power handling limits.
A sealed enclosure uses the trapped air as a spring that restores the cone after each stroke. The result is tight, accurate bass with a gentle 12 dB/octave rolloff below resonance. Sealed boxes are smaller, more forgiving of build tolerances, and ideal for music-focused systems that prioritize sound quality over raw output.
A ported box adds a tuned vent that reinforces output near the port tuning frequency, extending low-end response by 3–6 dB compared to sealed. The tradeoff: a steeper 24 dB/octave rolloff below tuning, larger cabinet volume, and the risk of port noise if air velocity gets too high. Port diameter, length, and flare geometry all affect performance — this calculator models port compression and chuffing thresholds so you can size vents correctly.
Bandpass enclosures combine sealed and ported chambers to create a natural acoustic filter. Output is limited to a defined passband, which can produce very high SPL in a narrow frequency range. Bandpass designs are sensitive to chamber ratios and port tuning — even small errors shift the passband or create peaks. Use the calculator's response graph to verify flatness before cutting wood.
The Efficiency Bandwidth Product (EBP) is a quick screening ratio: EBP = Fs / Qes. It tells you whether a driver is optimized for sealed or ported operation before you run full modeling.
EBP is a starting point, not a verdict. Drivers in the 50–90 range benefit most from full T/S modeling — enter your parameters above and compare the sealed and ported response curves side-by-side.
Every subwoofer box design starts with four core Thiele-Small (T/S) parameters published on the driver's datasheet. Here's what they mean and why they matter for enclosure design:
The frequency at which the driver cone naturally resonates in free air. Lower Fs generally indicates a driver suited for deep bass extension. A 15" competition subwoofer might have Fs around 25–35 Hz, while a compact 8" driver could sit at 40–55 Hz.
Qts combines electrical damping (Qes) and mechanical damping (Qms) into a single number describing how the driver behaves at resonance. Lower Qts (0.2–0.4) typically suits ported boxes; higher Qts (0.4–0.7) favors sealed designs. Qts above 0.7 may indicate the driver needs a very large sealed enclosure or is designed for free-air/infinite-baffle use.
Vas is the volume of air that has the same "springiness" as the driver's suspension. Larger Vas means a more compliant cone that needs a bigger box. A driver with Vas of 80 L will need significantly more enclosure volume than one with Vas of 20 L. This parameter is the biggest factor in determining final box size.
The distance the cone can travel in one direction before distortion becomes significant. Xmax determines the driver's displacement volume (Vd = Sd × Xmax) and ultimately its maximum clean output. Competition subwoofers often exceed 25 mm of Xmax; daily drivers typically sit around 8–15 mm.
Where to find T/S specs: Check your subwoofer's product page on Sparked Builds — we pull verified specs from manufacturer datasheets and community measurements. You can also click "Use in Box Calculator" on any subwoofer product page to pre-load its parameters automatically.
Port tuning frequency, diameter, and length are interdependent. A longer port tunes lower; a wider port handles more airflow before chuffing. This calculator models port air velocity in real time — if the velocity indicator turns red, increase the port diameter or add a second port.
For multi-driver ported enclosures, total port area should scale with the number of drivers. Two 12" subs need roughly twice the port cross-section of a single 12" — not twice the port length.
Design sealed and ported enclosures using Thiele-Small parameters. Select a driver from our database or enter specs manually.
Search our database of subwoofers with T/S parameters
Calculate both sealed and ported options to see which enclosure type is recommended for this driver based on its EBP (Efficiency Bandwidth Product).
Search for your subwoofer or enter T/S specs manually
Choose enclosure type (Sealed, Ported, or Compare)
Click Calculate to see results
A sealed (acoustic suspension) enclosure is airtight and produces tight, accurate bass with a smooth rolloff. A ported (bass reflex) enclosure has a tuned port that extends low-frequency output, producing louder bass at the tuning frequency but with a steeper rolloff below it. Sealed boxes are typically smaller and easier to build, while ported boxes are larger but more efficient.
Thiele-Small (T/S) parameters are electromechanical measurements that describe a speaker driver's behavior. The three critical ones are Fs (free-air resonance frequency), Qts (total Q factor combining electrical and mechanical damping), and Vas (equivalent compliance volume). These values determine the optimal enclosure size and type for your specific subwoofer.
Calculate the EBP (Efficiency Bandwidth Product) by dividing Fs by Qes. An EBP below 50 suggests sealed, above 100 suggests ported, and between 50-100 means either will work well. Drivers with low Qts (under 0.4) and low Fs typically perform best in ported enclosures, while higher Qts drivers often sound better sealed.
Qtc is the total system Q factor of the driver in its sealed enclosure. A Qtc of 0.707 (Butterworth alignment) gives the flattest possible response. Lower values (0.5-0.6) sound tighter with better transient response but less bass extension. Higher values (0.8-1.1) add a bass bump that sounds punchier but less accurate. For car audio, 0.707 is the standard recommendation.
Port length depends on the box volume, tuning frequency, and port cross-sectional area. This calculator handles the math using the standard port resonance formula. As a rule of thumb: larger ports need longer lengths, lower tuning frequencies need longer ports, and larger boxes need longer ports. Always ensure minimum port diameter of 3" for 10" subs, 4" for 12" subs, and 6" for 15" subs to avoid port noise.
Round (aero) ports are easier to build and have less turbulence at high output levels. Slot ports can be integrated into the box walls, saving internal volume and allowing for larger port area. For most builds, round ports with flared ends are the best balance of performance and simplicity. Slot ports are preferred when space is tight or when you need very large port area for SPL competition builds.