Sealed Box Calculator

Q: How do you calculate sealed box volume?

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 sit below the target Qtc — no box can reduce a driver's native Q.

Q: What is the best Qtc for a sealed subwoofer?

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

Q: How do I know if my sealed box is leaking?

Run the push test: press the cone in gently and release. In a sealed box it should return slowly over 1–2 seconds. Instant return means air is escaping — check the terminal cup, driver gasket, and panel joints.

Q: Do sealed subwoofers need a subsonic filter?

Generally no. The trapped air spring controls the cone at all frequencies, so there is no unloading like in ported boxes. Excursion still rises as frequency falls, but gently and predictably.

Q: Sealed or ported: which sounds better for music?

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.

Q: Can a sealed box be too big?

Yes. Oversized boxes push Qtc toward 0.5 and below — overdamped, thin bass, wasted space. Bigger is not better; the target Qtc dictates the volume. A huge result means this driver wants a smaller home.

Q: What thickness MDF should I use?

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

Key Takeaways

Smaller box raises Qtc; larger box lowers it: box volume is Vb = Vas / ((Qtc/Qts)² − 1).
Qtc 0.707 (Butterworth) gives the flattest response; 0.577 (Bessel) gives the fastest transients; 0.9+ sounds warm and punchy; 0.5 is critically damped.
Fc = Fs × (Qtc/Qts) — system resonance always sits above the driver's free-air Fs.
Polyfill makes the box behave 15–20% larger — a real tuning tool, not just damping.
Usable bass (F10) extends below F3: the gentle 12 dB/octave roll-off sums with room gain.
No port math and no subsonic filter — the trapped air spring protects the driver at all frequencies.

What Is a Sealed Enclosure?

Definition: A sealed (closed-box, 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 cannot run away below resonance the way it does in a ported box, so sealed subs survive without subsonic filters.

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

How This Calculator Works

This calculator solves the sealed-box equations from your driver's Vas, Qts, and Fs plus your target Qtc.

It outputs net box volume (convertible between liters, ft³, and in³), the resulting system resonance Fc, the F3 cutoff, and a plain-English damping label for your Qtc choice.

Methodology: Vb = Vas / ((Qtc/Qts)² − 1); Fc = Fs × (Qtc/Qts); F3/Fc = √((a + √(a² + 4)) / 2) with a = 1/Qtc² − 2.

Qtc labels: 0.707 Butterworth, 0.577 Bessel, below 0.6 Tight, above 0.9 Warm. We checked the widget's output against hand calculations for 5 driver profiles; every result matches to the displayed digit.

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.
With a = 1/0.707² − 2 ≈ 0, F3/Fc = √((0 + 2)/2) = 1 → F3 ≈ 53 Hz. The Butterworth −3 dB point coincides with Fc, as theory demands.

The pattern holds at other alignments: at Qtc = 1.0, a = −1, so F3/Fc = √((−1 + √5)/2) ≈ 0.786.

A peaky alignment trades deep extension for a hump near resonance — the −3 dB point sinks below Fc.

Tightening to Qtc = 0.577 needs 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. It must sit below your target Qtc, or the math goes negative — no box can lower a driver's native Q.
Driver Fs (Hz)
Free-air resonance — the starting point Fc builds on.
Target Qtc
Your damping choice — see the table below. 0.707 is the safe default.
Volume unit
Liters, cubic feet, or cubic inches — match whatever your saw measures in.

Reading Your Result

Qtc	Character	Box size	Choose it for
0.50	Critically damped, lean	Very large	Measurement and reference monitors
0.577	Bessel — fastest transient	Large	Music-first systems, audiophile bass
0.707	Butterworth (B2) — 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

A naming correction: some calculator sites label Qtc 0.5 as "Butterworth" and 0.707 as "Chebyshev".

The standard names run the other way — 0.707 is the Butterworth (B2) alignment (maximally flat), 0.5 is critically damped, 0.577 is Bessel, and 1.0+ behaves like a peaked Chebyshev. This page and the widget use the standard names.

Polyfill: The 15% Bigger Box

Polyester fiberfill makes the box behave up to 15–20% larger. The stuffing slows sound inside the enclosure, so the driver sees a bigger air spring: Fc drops, Qtc falls slightly, bass extends.

Use it as a design tool: shrink a box that came out too big to build, or nudge Qtc downward without rebuilding. The dose is about 1 lb per ft³, kept fluffy and even.

Packed-solid fill restricts air movement and raises Q instead.

The 60-Second Airtightness Audit

A sealed box that leaks is a badly tuned ported box. Leaks raise effective Q unpredictably and thin out low bass. Audit yours before trusting the math:

The push test: press the cone in gently and release — it should return slowly, over 1–2 seconds, as air seeps back through the dust-cap vent. Instant return means air is escaping.
Usual suspects: terminal cup seals, the 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 hot glue over wire holes).

Your Usable Bass Extends Below F3

F3 marks −3 dB, not silence. Sealed boxes roll off at only 12 dB/octave, so output one octave below F3 sits roughly 12 dB down — still audible and still EQ-able.

Builders call the −10 dB point F10 the practical bass limit.

Then the room helps: a typical living room adds bass below ~40 Hz (room gain), and a car cabin adds up to 12 dB/octave below 50–70 Hz.

A sealed sub measuring F3 = 50 Hz in free air routinely plays flat to 30 Hz in a car. Judge a sealed design by F10 plus the room, never by F3 alone.

Limitations

Small-signal model. At high excursion, suspension nonlinearity and voice-coil heating shift the effective Qts.
No leakage modeled. Real boxes leak a little; the math assumes perfect sealing — run the audit above.
Driver parameters vary. Published Vas and Qts are nominal; measure your actual driver for critical builds.
Room not included. Room gain extends real-world bass well below the calculated F3 — see the F10 section.

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 sit below the target Qtc — no box can reduce a driver's native Q.

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

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

Run the push test: press the cone in gently and release. In a sealed box it should return slowly over 1–2 seconds. Instant return means air is escaping — check the terminal cup, driver gasket, and panel joints.

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 shows which side you are on before you cut wood.

Generally no. The trapped air spring controls the cone at all frequencies, so there is no unloading like in ported boxes. Excursion still rises as frequency falls, but gently and predictably.

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.

Yes. Oversized boxes push Qtc toward 0.5 and below — overdamped, thin bass, wasted space. Bigger is not better; the target Qtc dictates the volume. A huge result means this driver wants a smaller home.

F3 is the frequency where output drops 3 dB below the midband — the nominal bass limit. For Qtc = 0.707, F3 equals the system resonance Fc. Usable bass (F10) extends further down, especially with room gain.

3/4-inch (19 mm) MDF is the standard for subwoofer boxes up to ~2 ft³. Use double-thickness baffles for heavy drivers and internal bracing for larger boxes. Panel flex steals bass — 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.

Sealed Enclosure Specifications

Physical Enclosure Dimensions (Reference Cube)