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Sealed box calculator: Qtc, Fc and frequency response for a subwoofer enclosure
Free Tool · Sealed Enclosure Designer

Sealed Box Calculator

Enter your subwoofer’s Thiele–Small parameters and box volume to compute Qtc, Fc, the −3 dB point, and a live frequency-response curve — then compare sealed alignments from Bessel to Butterworth.

CalculatorWhat is QtcRead Your ResultReal ExamplesWhat You NeedFAQ
Quick answer: A sealed box calculator sizes a closed (sealed) subwoofer enclosure from the driver’s Thiele–Small parameters. Enter Qts, Fs, Vas and your box volume Vb, and it returns system Q (Qtc), system resonance (Fc), and the response curve. Aim for a Qtc near 0.707 for the flattest, most accurate bass.
Expert-built Built & reviewed by Scott Welch — IASCA & MECA Sound Quality Champion with 40+ years in car audio.

Sealed Box Calculator

Sealed enclosure designer — enter four Thiele–Small values (or pick a sub we carry) to compute Qtc, Fc, and frequency response.

Qtc, Fc & frequency response

… or load an example driver:
–Qtc
–Fc (Hz)
–Box ratio (Vas/Vb)
–−3 dB (Hz)
–

Frequency Response (relative SPL, normalized)

Your boxCompare

Optimal Vb Targets — click a row to load it

AlignmentQtcVbFc−3 dBCharacter
Calculate to see suggested alignments for this driver.
Formula reference
Box ratio (a)   a = Vas / Vb
System Q   Qtc = Qts × √(a + 1)
System resonance   Fc = Fs × √(a + 1)
−3 dB point   solved from |H(f)| = 1/√2
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What Is Qtc — and What Makes a Good Sealed Box?

A sealed box (also called a closed or acoustic-suspension enclosure) traps a fixed volume of air behind the subwoofer. That trapped air acts as a spring, adding stiffness that raises the driver’s effective Q and resonance. The result is Qtc — the total system Q of the driver in that specific box — and Fc, the system resonant frequency.

Qtc = Qts × √(Vas / Vb + 1)

Smaller boxes raise Qtc (tighter, punchier, more peak); larger boxes lower it (deeper, more accurate, bigger cabinet). Qtc is the single number that tells you how a sealed enclosure will sound, which is why it’s the target every sealed box calculator solves for.

How to Read Your Qtc

Use your calculated Qtc to judge the sealed alignment. Higher Qtc = smaller box, more peak; lower = bigger box, deeper and flatter.

AlignmentQtcVbFc−3 dBCharacter
Critically Damped0.50047.9 L46.1 Hz46.1 HzUltra-smooth, no ringing. Very extended −3 dB. Needs a large box.
Bessel B20.57726.8 L53.1 Hz53.1 HzBest transient step response. Low overshoot. SQ-oriented.
Maximally Flat0.70714.2 L65.1 Hz65.1 HzButterworth B4. Widest flat bandwidth. Ideal all-around.
Warm / Punchy0.9007.6 L82.9 Hz68.8 HzSlight bass boost around Fc. Common in car audio for impact.
Bass Boost1.0005.9 L92.1 Hz72.4 HzNoticeable peak. Smaller box, boosted midbass. May ring.

Figures shown are worked for a representative SQ driver (Qts 0.38, Fs 35 Hz, Vas 35 L) — your calculator above uses your driver’s own values. Note that Qtc must be higher than the driver’s Qts — the box can only add Q, never remove it. If your target Qtc is below Qts, the driver wants a bigger box or a ported enclosure.

Worked Examples: Real Subs We Carry

Published Thiele–Small data run through the sealed box calculator at a 0.707 Butterworth target.

SubwooferQtsFsVasVb for 0.707
Arc Audio ARC Series 100.3923.8 Hz45.8 L≈ 0.71 ft³
Eton PW120.4631.6 Hz43.7 L≈ 1.15 ft³
Image Dynamics ID8 V4 (8″)0.4631 Hz26.0 L≈ 0.69 ft³
Audiomobile Encore 44120.5232 Hz30.1 L≈ 1.23 ft³

Pick any of these (and dozens more) from the dropdown in the calculator to auto-fill its parameters, then read the exact Vb for your target. Browse our subwoofers.

Sealed vs Ported — and Does a Sealed Box Need Tuning?

A sealed enclosure is smaller, tighter, and more accurate; a ported box is louder and digs deeper but needs more space and precise port tuning. Not sure which your driver wants? Run the EBP calculator first — below about 50 favors sealed, above about 100 favors ported.

One common mix-up: a sealed box has no port to tune, so there is no “tuning frequency” like a ported box. The equivalent target for a sealed design is your Qtc — set it by choosing the box volume above. Port tuning only applies to ported builds.

What You’ll Need: Thiele–Small Parameters

The sealed box calculator runs on four values from your subwoofer’s spec sheet:

QtsTotal Q of the driver — combines electrical and mechanical damping.
FsFree-air resonant frequency, in Hz.
VasEquivalent compliance — the air volume with the same springiness as the suspension.
VbYour net internal box volume — the air the driver actually sees, after displacement.

Bought your sub from us? Send us the model and we’ll pull the Thiele–Small parameters for you.

Finish the Build with the Rest of the Suite

The sealed box calculator sizes the enclosure. These tools take you the rest of the way:

EBP CalculatorSealed or ported? Decide first Ported / Port LengthIf your driver wants a port Displacement HelperNet → gross build volume

Rather We Just Build It?

We CNC-cut Proline X sealed enclosures to your driver’s exact Qtc target — spec-matched, built in Tennessee.

Shop Proline X Enclosures Open the Calculator Suite

Sealed Box Calculator FAQ

How do I use a sealed box calculator?
Enter your subwoofer’s Qts, Fs, and Vas from its spec sheet, plus your planned net box volume (Vb). The calculator returns Qtc, Fc, the −3 dB point, and a frequency-response curve, and suggests the box volume for each sealed alignment. You can also pick a sub we carry to auto-fill the Thiele–Small values.
What is the best Qtc for a sealed subwoofer box?
A Qtc of 0.707 (Butterworth) is the flattest, most accurate response and the usual sound-quality target. 0.577 (Bessel) gives the best transient response but needs a larger box; 0.8 to 1.0 shrink the box and add punch with a rising peak. Match the number to your goals and space.
What size sealed box does my subwoofer need?
It depends on the driver’s Qts, Fs, and Vas and your target Qtc. Enter those values and the calculator solves the exact net volume (Vb) for each alignment in the Optimal Vb table — click a row to load it. As a rule of thumb, lower-Qts SQ drivers want larger sealed boxes; higher-Qts drivers suit smaller ones.
Does a sealed box need a tuning frequency?
No. A sealed (closed) box has no port, so there is no tuning frequency to set — that concept belongs to ported enclosures. The sealed equivalent is your Qtc, which you control by choosing the box volume. If you searched for a “sealed box tuning calculator,” Qtc and Fc are the numbers you actually want.
What Thiele–Small parameters do I need?
For a sealed design you need Qts (total Q), Fs (free-air resonance), and Vas (equivalent compliance), plus your box volume Vb. These are on the manufacturer’s spec sheet or product page. This tool doubles as a Thiele–Small parameters calculator: give it the driver values and it converts them into Qtc, Fc, and response.
What’s the difference between a sealed and ported box?
Sealed is smaller, tighter, and more accurate with better transient response; ported is louder and extends deeper but needs more volume and precise port tuning. Use the EBP calculator to decide which alignment your specific driver was designed for before you size the box.
Can I use this for a speaker box, not just a subwoofer?
Yes. The math is the same for any driver you plan to seal in a box — it works as a sealed speaker box calculator too, as long as you have the driver’s Qts, Fs, and Vas. It’s tuned for car-audio subwoofers, but the Qtc and Fc results apply to any sealed enclosure.
What is a closed box or acoustic-suspension enclosure?
“Closed box,” “sealed box,” and “acoustic suspension” all mean the same thing: an airtight enclosure where the trapped air acts as a spring on the cone. This calculator is effectively a closed-box simulator — it models how that sealed air changes the driver’s Q and resonance.
How do I find my box’s internal volume?
Multiply internal width × height × depth (in inches) and divide by 1,728 for cubic feet, then subtract the volume taken up by the driver, port, and bracing to get net volume. Enter that net figure as Vb. Our displacement helper in the suite converts a net target into the gross volume you should build to.
Is a sealed enclosure better for sound quality?
For most sound-quality builds, yes — a sealed box at Qtc 0.707 or lower gives flat, accurate, well-controlled bass with excellent transient response, and it’s more forgiving of placement and box tolerances than a ported design. Ported wins when you need maximum output and deep extension over outright accuracy.
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Sealed box calculator by Audio Intensity.