Boxsmith: Subwoofer Box Design

Subwoofer box design with a real Thiele-Small simulation, port sizing and a cut sheet.

Boxsmith is a subwoofer box calculator for Android that simulates how your driver will actually play, the way desktop box-design tools do. Enter the driver's Thiele-Small parameters, choose a sealed, ported or bandpass box, and see the response, cone excursion and port air speed before you cut any wood.

Then it sizes the port, fits the box to the space in your trunk or room, and draws a cut list and a sheet layout you can take to the shop as a PDF. Every feature is free, with no account and no paywall on the results.

Screenshots

Why we built it

Calculators that only give a volume. Boxsmith simulates the whole system: response, maximum output, excursion against Xmax, port air speed, impedance and group delay.
Results behind a paywall. Every graph, suggestion and cut sheet is free. An optional one-time purchase only removes the ads.
Ports that chuff or don't fit. Port length includes the end correction, one tap sizes the port under an air-speed limit, and Boxsmith tells you when a port needs a bend.
Spec-sheet typos that ruin a design. Every driver value is cross-checked: a Qts that disagrees, Vas typed in litres instead of cubic feet, or an Xmax that looks peak-to-peak.

Features

Simulation, not just a volume

Full Thiele-Small model of the driver and box, the same family of model WinISD uses
Graphs: response (SPL), maximum SPL, cone excursion against Xmax, port air speed, impedance and group delay
Set the amplifier power, a subsonic filter, a low-pass, bass boost and car cabin gain, and see the effect at once
Overlay up to four designs to compare them
Tap or drag any graph to read exact values

The right box for your sub

Sealed, ported and 4th-order bandpass boxes
"Suggest a box" finds flat, deeper-bass and compact designs for your driver with the simulation, keeping the port buildable
Warnings in plain words: past Xmax, a port that will chuff, a load too low for the amp, more power than the drivers are rated for
Multiple subs in one box, single or dual voice coils, every wiring option with its final load

Ports done properly

Slot or round ports, one or several, with the end correction included
Port length for any tuning; one tap sizes the port to stay under the air-speed limit
Tells you when the port needs an L-shaped bend

Build it

Enter the width and height that fit your trunk or room and fit the depth to the volume
Wedge (sloped back) boxes for trucks and hatchbacks
Net volume after the drivers, port and bracing are taken out, checked against the design
3D preview with the drivers and port on the baffle
Cut list in inches and fractions or millimetres, double baffle, port panels
Sheet layout on 4 × 8 ft or metric sheets with your saw kerf
Cut sheet as a PDF to take to the shop

Your drivers

Type the T/S parameters from the spec sheet in any common units; Boxsmith works out the rest (Qts, Mms, Cms, Bl, efficiency, sensitivity, EBP)
Every value is cross-checked: it warns when Qts disagrees, when Vas looks like cubic feet typed as litres, or when Xmax looks peak-to-peak
Share a driver as a file with a friend; import theirs
Example drivers to try it straight away

Tools

Subwoofer wiring calculator for every combination of subs and voice coils
Port length and port tuning calculator
Box volume calculator, including wedges
Quick sealed box from Fs, Qts and Vas
Passive crossover values (1st to 4th order, Butterworth and Linkwitz-Riley), L-pad and Zobel

How it works

Boxsmith uses the standard lumped-parameter (Thiele-Small) model of a driver in a box, the same family of model desktop tools such as WinISD use, with air at 25 °C. You can type any common subset of the driver's parameters; the rest (Qts, Mms, Cms, Bl, efficiency, sensitivity, EBP) are derived and cross-checked.

Box suggestions start from the classic alignments: closed-box formulas for sealed boxes, and a search with the simulation for ported and bandpass boxes, narrowing the port when needed so its length stays buildable. The engine's tests check it against closed-form results, including Small's closed-box Qtc, fc and f3, Thiele's B4 vented alignment and standard crossover tables.

The build step fits the depth to the net volume after the drivers, port and bracing are taken out, so the box you cut has the volume you designed.

Honest limits

The simulation is a standard idealised model; real boxes, drivers and cars differ a little. Check your build before you cut.
Results are only as good as the driver's published parameters. Measured parameters from your own driver will always be more accurate than a spec sheet.

Questions

What is the best free subwoofer box calculator app?

Boxsmith is a free Android app that simulates sealed, ported and bandpass boxes with the full Thiele-Small model, sizes the port and produces a cut sheet. Nothing is behind a paywall and it works offline.

How do I calculate a ported subwoofer box?

Enter your driver's T/S parameters, choose a ported box, and use “Suggest a box” or set the volume and tuning yourself. Boxsmith simulates the response and excursion, sizes the port for the tuning while keeping air speed under the limit, then fits the box dimensions to the net volume.

How long should my subwoofer port be?

Port length depends on the box volume, the tuning frequency and the port's area, plus an end correction. Boxsmith's port calculator works it out for slot or round ports, one or several, and warns when the port is too long to fit straight.

Sealed or ported: which is better?

Sealed boxes are smaller and have gentler roll-off and lower group delay; ported boxes play louder and deeper near the tuning frequency but need more space and a well-sized port. Boxsmith lets you overlay up to four designs to compare them for your driver.

How do I wire two dual voice coil subwoofers?

Boxsmith's wiring calculator lists every combination of subs and voice coils with the final load, so you can match it to what your amplifier is stable at.

Does Boxsmith work in metric?

Yes. Use inches and cubic feet or centimetres and litres; cut lists come in inches and fractions or millimetres.

Is Boxsmith free?

Yes. Every feature is free with ads, and an optional one-time purchase removes them; there is no subscription.

Private and offline

Runs entirely on your phone and works offline. No account, no sign-up, and your data stays on your device. The app shows ads through Google AdMob; see the privacy policy. Privacy policy.

Free. An optional one-time purchase removes the ads; nothing is locked and there is no subscription.

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