Which Thiele-Small parameters do I need?

For first-cut enclosure design you need three: Fs (Hz), Qts, and Vas (litres or ft³). For port velocity / Xmax checks also Sd and Xmax. Re, Le, Bl, Mms etc. are derived parameters and don’t change the box design.

Why doesn't my in-room bass match this plot?

This plot is anechoic. In-room you get boundary gain (3–9 dB below 80 Hz), room modes (standing-wave peaks/dips), and the system high-pass. Use this for cabinet design, then measure and EQ in-room.

What is an isobaric subwoofer enclosure?

An isobaric (or compound) enclosure pairs two identical drivers in a sealed inner chamber in opposition (push-pull). The system behaves as one driver with half the Vas, so the required box volume for a given Qtc is halved. The trade-off: you need two drivers for the same output as one driver in a standard box. Isobaric is useful when cabinet volume is the hard constraint.

💣

Subwoofer Frequency Calculator

Enter your driver's Thiele-Small parameters (Fs, Qts, Vas) and the tool computes the optimal sealed or ported enclosure: box volume, tuning frequency, −3 dB point, port length with end correction, and a response plot. Sealed math is exact; ported uses the well-known QB3/B4/C4 regression formulas.

Input

Driver Fs (free-air resonance, Hz)

From the driver's spec sheet. Typical subs: 18–35 Hz.

Driver Qts (total Q at Fs)

Qts = Qes·Qms / (Qes+Qms). Sealed-friendly drivers: 0.4–0.7.

Driver Vas (equivalent compliance volume)

Target system Qtc 0.707

0.5 = Bessel (max-damped) · 0.707 = Butterworth (flat) · 1.0+ = peaked. The classic target is 0.707.

Result

—

Formulas & reasoning

—

Anechoic frequency response (10–500 Hz)

Thiele-Small & Enclosure Basics

Three driver parameters from the spec sheet determine everything a subwoofer box calculator needs: Fs (the driver's free-air resonance frequency in Hz), Qts (its total Q at Fs), and Vas (its equivalent compliance volume — the volume of air whose acoustic compliance equals the driver's mechanical compliance). Plug those in and the math tells you what box to build and where the system's −3 dB point (F3) will land.

Sealed vs ported — which to choose?

Sealed boxes (also called acoustic suspension) have a 2nd-order high-pass rolloff at 12 dB/oct — gradual, with excellent transient response. They're small for the bass extension they give and forgiving of driver Qts (anything ≈ 0.3–0.7 works). The trade-off is lower SPL output for the same driver.

Ported boxes (also called a vented enclosure or bass-reflex cabinet) add a Helmholtz resonator (the port) that reinforces output around its tuning frequency Fb. The response rolls off at 24 dB/oct below Fb — steep, with deeper bass extension and more SPL than sealed, but worse transient response and unloading below tuning (driver can over-excurse there). Ported wants Qts ≈ 0.25–0.45 for a clean alignment.

The EBP rule of thumb

If you don't know which type to build, compute EBP = Fs / Qes (Efficiency Bandwidth Product). EBP < 50 → sealed is best. EBP > 100 → ported is best. 50–100 → either works. EBP is a quick proxy for motor strength relative to compliance: a high-EBP driver has a stiff motor that can handle the unloading below port tuning, while a low-EBP driver's suspension does the work — it suits the acoustic suspension (sealed) loading better. You can also think of it as: high EBP = suitable for bass-reflex alignments; low EBP = isobaric or sealed.

The Qtc dial (sealed)

In a sealed box the driver's Qts is raised by the box's acoustic stiffness to a new value, Qtc (system Q). Designers pick a target Qtc and the math gives back the box volume needed: α = (Qtc/Qts)² − 1, Vb = Vas/α. Qtc = 0.707 (Butterworth) is the classic flat-response target; lower Qtc (e.g. 0.5) damps the bass for tighter transients but at the cost of bass extension; higher Qtc (e.g. 1.0) gives a peaky bump that extends perceived bass at the cost of accuracy. The slider in the tool lets you sweep Qtc and watch Vb / Fc / F3 update live.

Why no "fourth-order" sealed?

A sealed box is intrinsically 2nd-order (the driver's mass + the box's stiffness form a single mechanical resonance). You can't make it 4th-order without adding electronic equalization (e.g., Linkwitz transform) or adding a port (vented box). The 4th-order Butterworth alignment is the B4 vented alignment — that's what the "Ported" tab here computes.

Port length and end correction

The port is a Helmholtz resonator: tube of cross-section Sp, length Lp, coupled to box volume Vb. The resonance frequency is Fb = (c/2π) · √(Sp / (Vb · Lp_eff)), where Lp_eff is the effective length — the physical tube length plus an "end correction" because air sloshing in/out of each end behaves as if there's a bit of extra tube there. For one flush end (against the cabinet wall) and one free end (sticking inside the cabinet), the correction is about 0.732 · √(Sp/π), or roughly 1.46 × the port radius. The tool subtracts that correction so the number it gives is what you cut.

Slot ports vs round ports

Round PVC ports are cheap and easy, but long ports are awkward to fit. A slot port — a rectangular duct that runs along the inside wall of the cabinet — can fold a 90 cm port into a box that's only 40 cm deep. The equivalent round-port diameter for a slot port is calculated from the cross-sectional area: a 15 × 5 cm slot has an area of 75 cm² ≈ 9.77 cm equivalent diameter. For slotted ports, add one end-correction term (the inner wall acts as a flanged termination). Slot ports also have lower port velocity for a given area, which pushes the chuffing onset higher — a real advantage in high-SPL designs. If you want to integrate a passive crossover into the cabinet, slot ports free up face-panel area for driver and terminal placement.

What this tool doesn't do

The plot is an anechoic estimate. In a real room you get boundary gain (3–10 dB below 80 Hz from the floor and walls), and a ported box's actual response depends on box losses (Ql), driver Le, and the port's air velocity (turbulence above ~17 m/s causes port noise, commonly called chuffing). For a finished design, simulate the box in WinISD / BassBox / Hornresp with the full T/S parameter set, then measure with a UMIK-1 + REW once it's built. To then set the crossover between this sub and your main speakers, use the subwoofer crossover optimizer.

Frequently Asked Questions

My driver's spec sheet has 20 parameters. Which do I need?

For first-cut enclosure design you only need three: Fs (Hz), Qts (dimensionless), and Vas (litres or ft³). For port velocity / Xmax checks you'd also want Sd (cone area) and Xmax (peak linear excursion). Re, Le, Bl, Mms, Cms etc. are computed from those primary parameters and don't change the box design.

Why does the recommended Vb for a ported box come out so small (or so large)?

The Dickason formula Vb = 15·Vas·Qts^2.87 is very sensitive to Qts. A Qts of 0.30 → Vb = 0.43·Vas (small box); Qts of 0.45 → Vb = 1.62·Vas (large box). Outside the QB3/B4/C4 zone (Qts < 0.25 or > 0.55) the formula extrapolates badly. If the Vb seems wrong, double-check Qts on the spec sheet, or use sealed instead.

Should I use sealed or ported?

EBP = Fs / Qes < 50 → sealed; > 100 → ported; in between either works. Sealed sounds tighter, fits in smaller boxes, and is hard to wreck with a wrong port. Ported gives more SPL and deeper extension but unloads below tuning (the driver can over-excurse if you drive it hard at very low frequencies). For music, sealed is often the safe default; for movies/LFE, ported gives more output where it matters.

Why is my port so long?

Port length grows with smaller diameter and lower Fb. A 6 cm port tuned to 20 Hz in a 50 L box is over a metre long. Solutions: use a larger-diameter port (length drops quadratically with diameter), bend the port (PVC elbows work for round; "slot ports" along the cabinet's inside wall save space), or accept that the box has to be bigger.

Can I just use the spec-sheet's "recommended box volume"?

Yes if it exists — manufacturers usually publish "sealed box" and "vented box" recommendations that they've optimised for. The math here will give you values close to those for sane Qts drivers. The advantage of computing yourself is you can choose a non-default Qtc (e.g., 0.5 for max damping, 0.8 for extended bass) and see what trade-off you get.

What about bandpass enclosures (4th- and 6th-order BP)?

Bandpass cabinets pack a driver between two chambers (one sealed, one ported, or both ported) for very high SPL in a narrow band. They have more design degrees of freedom and aren't covered here. Use a dedicated simulator (WinISD, Hornresp) — bandpass is also unforgiving of design errors, with serious risk of overdriving the driver if you get it wrong.

My in-room bass doesn't match this plot. What gives?

This plot is anechoic — what you'd measure in an empty echo-free chamber. In your room you get boundary gain (placement against floor/wall/corner adds 3, 6, or 9 dB below 80 Hz respectively), room modes (standing waves boost/cut specific bass frequencies — see our Subwoofer Crossover Optimizer), and the system's actual high-pass crossover. Use this tool for the cabinet design, then measure in-room and apply EQ as needed.

What are Xmax and Xmech, and do they affect box design?

Xmax is the peak linear excursion — how far the cone can travel before the motor starts losing force (Bl drops). Xmech is the absolute maximum travel before the spider or surround hits a hard stop. Neither changes the enclosure volume or tuning frequency calculated here, but they set a power limit: in a ported box below tuning (Fb), the driver rapidly unloads and excursion shoots up. High-output home theatre builds always check that the driver's Xmax isn't exceeded at the target listening level below Fb. If your manufacturer lists Xmax ≤ 10 mm and you're planning 105 dB at 20 Hz, reconsider the driver or add a high-pass filter below Fb.

How do I measure Thiele-Small parameters myself if I don't have a spec sheet?

The most common method is the "added mass" or "added volume" technique using a free-ware measurement suite such as REW or DATS V3. You measure the driver's impedance curve with a known small added mass on the cone, then again without — the shift in the resonance peak lets you back-calculate Fs, Qts, and Vas. Results are only as accurate as your mass measurement and test setup, but they're usually close enough for enclosure design. Drivers also break in over the first 10–20 hours of use, which lowers Fs and Qts slightly, so measure after break-in for final designs.

What is an isobaric (push-pull) subwoofer enclosure?

An isobaric (or compound) enclosure pairs two identical drivers in a sealed inner chamber, breathing in opposition (one pushing while the other pulls). The combined system behaves as a single driver with half the Vas — meaning you can use half the box volume for the same Qtc as a single-driver sealed design. The trade-off: you need two drivers for the same output, so you use twice the cone area to get the same SPL as one driver in a standard sealed box. Isobaric alignment is useful when box volume is the hard constraint (e.g., custom car-audio installs), not when efficiency is the priority.

Related Tools

← All Signal Processing Tools