Sealed Box Calculator

Last updated: October 31, 2026 · Written by the Pitch Detectors Editorial Team

A sealed box calculator sizes an airtight speaker enclosure from your driver's Thiele/Small parameters. Enter Vas, Qts, Fs, and your target Qtc — this tool outputs the required box volume, system resonance (Fc), and −3 dB point (F3) in liters, cubic feet, or cubic inches.

Key Takeaways

Box volume: Vb = Vas / ((Qtc/Qts)² − 1) — smaller box raises Qtc.
Qtc 0.707 = flattest response; 0.577 = tightest transients; 0.9+ = warm and punchy.
System resonance: Fc = Fs × (Qtc/Qts) — always above the driver's free-air Fs.
Polyfill makes the box act up to ~15–20% larger.
Sealed boxes need zero port math and no subsonic filter — the air spring protects the driver.

The trapped air in a sealed box acts as a spring, raising system resonance and controlling the cone.

What Is a Sealed Enclosure?

Definition: A sealed (closed-box or acoustic suspension) enclosure is an airtight cabinet where the trapped air acts as a spring behind the driver cone. This air spring raises the system's resonance frequency and damping, producing tight, controlled bass that rolls off gently at 12 dB per octave below the −3 dB point.

Edgar Villchur's acoustic-suspension idea (1950s, Acoustic Research) turned the box from a necessary evil into part of the suspension: the air spring linearizes the driver's motion and protects it — excursion can't run away below resonance the way it does in a ported box. That's why sealed subs survive without subsonic filters.

The price is efficiency: sealed boxes roll off earlier than ported equivalents. But the gentle 12 dB/octave slope sums beautifully with room gain and responds well to EQ — which is why sealed remains the accuracy-first choice.

How This Calculator Works

This calculator solves the classic sealed-box equations from your driver's Vas, Qts, and Fs plus your chosen target Qtc. It outputs net box volume (convertible between liters, ft³, and in³), the resulting system resonance Fc, and the F3 cutoff, with a plain-English damping label for your Qtc choice.

Methodology: Vb = Vas / ((Qtc/Qts)² − 1); Fc = Fs × (Qtc/Qts); F3/Fc = √((1/Qtc² − 2 + √((1/Qtc² − 2)² + 4)) / 2). Qtc labels: 0.707 Butterworth, 0.577 Bessel, below 0.6 Tight, above 0.9 Warm. We verified the widget's math against hand calculations for 5 driver profiles; all outputs match to the displayed precision.

Formula + Worked Example

Vb = Vas / ((Qtc/Qts)² − 1) · Fc = Fs × (Qtc/Qts)

Worked example: driver with Vas = 50 L, Qts = 0.40, Fs = 30 Hz, target Qtc = 0.707:

Ratio: (0.707/0.40)² − 1 = (1.7675)² − 1 = 3.124 − 1 = 2.124.
Vb = 50 / 2.124 = 23.5 L (0.83 ft³).
Fc = 30 × 1.7675 = 53.0 Hz.
F3/Fc = √((1/0.707² − 2 + √((1/0.707² − 2)² + 4)) / 2) = √((0.0006 + 2.0000) / 2) ≈ 1.0002 → F3 = 53.0 × 1.0002 ≈ 53 Hz — the Butterworth −3 dB point coincides with Fc, as expected. (For comparison, Qtc = 1.0 gives a ratio of 0.786, so F3 sinks to 0.786 × Fc: a peaky alignment trades deep extension for output near resonance.)

Want it tighter? Target Qtc 0.577: Vb = 50 / ((1.4425)² − 1) = 50 / 1.081 = 46.3 L — nearly double the volume for the Bessel alignment.

How to Use This Calculator

Driver Vas (liters)
Equivalent compliance volume from the spec sheet — the air volume with the same stiffness as the suspension.
Driver Qts
Total Q of the driver. Must be below your target Qtc, or the math goes negative (no box can lower Q).
Driver Fs (Hz)
Free-air resonance — the starting point Fc builds on.
Target Qtc
Your damping choice — see the tasting menu below. 0.707 is the safe default.
Volume unit
Liters, cubic feet, or cubic inches — pick whatever your saw measures in.

Reading Your Result

Qtc	Character	Box size	Choose it for
0.50	Critically damped, lean	Very large	Measurement/reference monitors
0.577	Bessel — fastest transient	Large	Music-first systems, audiophile bass
0.707	Butterworth — flattest	Medium	The default; home theater, general use
0.9	Warm, slight lift	Compact	Car audio punch, small rooms
1.1	Peaky, one-note risk	Very compact	Maximum boom per liter; rarely hi-fi

Polyfill: The 15% Bigger Box

Stuffing the box with polyester fiberfill doesn't just damp internal reflections — it slows the speed of sound inside, making the driver behave as if the box were up to 15–20% larger. A 20 L box stuffed at ~1 lb per ft³ acts like ~23 L: Fc drops, Qtc falls slightly, bass extends.

This is a legitimate design tool, not a cheat: use it to shrink a box that came out too big to build, or to fine-tune Qtc downward without rebuilding. Don't overstuff — packed-solid fill restricts air movement and raises Q instead. Fluffy and even wins.

How Qtc shapes the response: higher Qtc = more lift at resonance, smaller box.

The Airtightness Audit

A sealed box that leaks isn't sealed — it's a badly tuned ported box. Leaks raise effective Q unpredictably and murder low bass. Audit yours:

The push test: gently push the cone in and release — it should return slowly (1–2 seconds) as air hisses back through the dust cap vent. Instant return = leak.
Usual suspects: terminal cup seals, driver gasket, unsealed screw holes, panel joints.
Fixes: closed-cell foam gasket tape under the driver, silicone or caulk on internal joints, sealed terminal cups (or seal wire holes with hot glue).

Limitations

Small-signal model. At high excursion, suspension nonlinearity and voice-coil heating shift effective Qts.
No leakage modeled. Real boxes leak a little; the math assumes perfect sealing.
Driver parameters vary. Published Vas/Qts are nominal — measure your actual driver for critical builds.
Room not included. Room gain extends real-world bass well below the calculated F3.

Sealed vs Ported at a Glance

	Sealed	Ported
Roll-off	12 dB/octave (gentle)	24 dB/octave below Fb (steep)
Transient response	Excellent	Good (more group delay near Fb)
Protection	Air spring; no subsonic filter needed	Unloads below Fb; filter required
Efficiency	Lower	+3–6 dB around Fb
Size	Compact	Larger

Compare: ported enclosure calculator.

Frequently Asked Questions

Use Vb = Vas / ((Qtc/Qts)² − 1). Example: Vas = 50 L, Qts = 0.40, target Qtc = 0.707 gives Vb ≈ 23.5 L. Your driver's Qts must be lower than the target Qtc — no box can reduce a driver's native Q.

0.707 (Butterworth) is the safe default — flattest response. Choose 0.577 (Bessel) for the tightest, most accurate transient response in music systems, or ~0.9 for a warmer, punchier character in cars. Above 1.0 gets peaky and one-note.

Yes — polyester stuffing slows sound inside the box, making the driver behave as if the volume were 15–20% larger. Use about 1 lb per cubic foot, kept fluffy. It's a real tuning tool for shrinking an oversized design, not just damping material.

Push the cone gently and release: in a sealed box it should return slowly over 1–2 seconds. Instant return means air is escaping somewhere — check the terminal cup, driver gasket, and panel joints. Even small leaks audibly thin the bass.

Qtc rises: bass gets peaky and one-note around Fc, with weak deep extension. Too large does the opposite — overdamped, thin, anemic bass. The widget's Qtc readout tells you which side you're on before you cut wood.

Generally no — the trapped air spring controls the cone at all frequencies, so there's no unloading like in ported boxes. Excursion still rises as frequency falls, but gently and predictably. This is one of sealed's practical advantages.

For accuracy, sealed: tighter transients, no port noise, gentle roll-off that room gain extends naturally. Ported wins on sheer output per dollar. A well-built sealed sub with modest EQ below F3 satisfies most music listeners completely.

Yes. Oversized boxes push Qtc toward 0.5 and below — overdamped, with thin bass and wasted space. Bigger is not better; the target Qtc dictates the volume. If the box came out huge, that's the math telling you this driver wants a smaller home.

The frequency where output drops 3 dB below the midband — the practical bass limit. For Qtc = 0.707, F3 equals the system resonance Fc. Because sealed rolls off gently at 12 dB/octave, usable bass (F10) extends well below F3, especially with room gain.

3/4-inch (19 mm) MDF is the standard for subwoofer boxes up to ~2 ft³; go double-thickness baffles for heavy drivers and add internal bracing for larger boxes. Panel flex steals bass and colors midrange — a well-braced 3/4" box beats a flimsy 1" one.

Box calculations use ideal Thiele/Small theory. Verify with a nearfield measurement of the finished enclosure.