How to Design a Subwoofer Box: Volume, Port, Plans and Cut Sheet

A sub in the wrong box sounds boomy, weak or just loud at one note. A box that matches the sub is the cheapest upgrade you can make. In this Instructable I go from the sub's spec sheet to a ported box that's ready to cut: picking the volume and tuning, checking the graphs, getting plans and a cut sheet that fits on one sheet of MDF, and checking the tuning of the finished box with a test tone.

As an example I use a popular 12" sub, the Sundown Audio SAv3-12 D2, in a ported box tuned to 33 Hz. The same steps work for any sub.

Full disclosure: I made the calculator used in step 3. It's free and doesn't need an account — you can do the same math in WinISD if you prefer.

1. One 4×8 ft sheet of 3/4" MDF (or plywood)
2. Wood glue, 1 5/8" screws, clear silicone
3. Terminal cup, 12–14 AWG speaker wire
4. Carpet or paint for the finish
5. Circular or table saw with a straight edge
6. Jigsaw or router with a circle jig
7. Drill/driver, clamps, square
8. Dust mask (MDF dust is nasty)
9. A free box calculator (I used Sub Box Calculator) or WinISD
10. A phone with a bass tone generator for checking the tuning

Every box calculation starts from the sub's Thiele/Small (T/S) parameters. You'll find them in the manual or on the manufacturer's page. The ones that matter most:

1. Fs — resonance frequency, Hz
2. Qts — how damped the sub is
3. Vas — equivalent air volume, ft³ or liters
4. Xmax — how far the cone can move

For the SAv3-12 D2: Fs 35 Hz, Qts 0.43, Vas 0.78 ft³, Xmax 0.87", dual 2 Ω coils, 1,500 W rated.

Tip: double-check the numbers against the official spec sheet. Some sheets list the cone area (Sd) as the full 12" circle, which is wrong for box math.

1. Sealed — smaller box, tight and accurate bass, rolls off earlier. Easy to build, very forgiving.
2. Ported — bigger box, more output around the tuning frequency, deeper bass for the same power. The port has to be the right size and length, or it chuffs (port noise).

A rough rule from Qts: under ~0.4 likes ported, over ~0.5 likes sealed, in between can do both. With Qts 0.43, the SAv3-12 works in either; I went ported for more low end.

If trunk space is tight, sealed is the safer choice. You can switch between the two in the calculator and compare.

I used Sub Box Calculator for this: https://subboxcalculator.com/calculator/

1. Click "Change sub" and search for your model — about 3,900 car subs are in the database. If yours isn't there, type in the T/S parameters from step 1.
2. Set Box to "Ported", enter your amp power (I used 300 W) and the outside size you can fit.
3. The calculator suggests the volume and tuning and sizes the port.

Result for the SAv3-12 D2:

1. Net volume: 1.94 ft³, tuned to 33 Hz
2. Port: 2" slot, L-shaped along the side and back wall
3. Outside size: 32" wide × 15" high × 12 5/8" deep, 3/4" MDF
4. F3 (−3 dB, without cabin gain): 29 Hz
5. Port air speed at 300 W: 49 ft/s — quiet (noise starts around 82 ft/s)

You can also check the manufacturer's recommended box on the "Manufacturer's advice" tab and compare it with the calculated one.

Before buying wood, look at three things:

1. Response — where the bass starts to drop off.
2. Cone excursion — the cone should stay under Xmax at your amp power, especially below the tuning frequency.
3. Port velocity — if it's high, the port will chuff. A bigger port area fixes it, but the port gets longer.

The "What if…" table shows what changes if you go 10% bigger or smaller, or tune 3 Hz higher or lower. For example, tuning 3 Hz lower makes the port about 7" longer and lowers F3 by 1.5 Hz.

The X-ray view shows where the port goes inside the box, so you can see that it doesn't hit the sub's magnet.

The "Plans" tab has front, side and top views, and the "Cut list" has every panel with its finished size. The outside dimensions on the plans are not the cut sizes — the baffle and back fit between the top and bottom, so they're shorter.

Cut list for this box (3/4" MDF, 8 pieces, one 4×8 ft sheet, about 50% used):

1. Top / bottom — 2 pcs — 32 × 12 5/8"
2. Baffle (Ø11 3/16" cutout, port on the left) — 1 pc — 29 1/4 × 13 1/2"
3. Back — 1 pc — 32 × 13 1/2"
4. Left side (port side) — 1 pc — 11 7/8 × 13 1/2"
5. Right side — 1 pc — 11 1/8 × 13 1/2"
6. Port wall along the side — 1 pc — 8 3/8 × 13 1/2"
7. Port wall along the back — 1 pc — 17 15/16 × 13 1/2"

Plans, cut sheet, PDF and DXF (for a CNC) are free to download. Recalculate for your own sub — these sizes are only for this design.

This Instructable is about the design, so here's the build in short. Everything you need is on the plans and the cut sheet from step 5. The finished box weighs about 42–48 lb.

1. Cut the big panels first, leaving 1/8" for the blade (the sheet layout already allows for it). Cut the Ø11 3/16" sub hole in the baffle, using the sub's template if it has one. Check every panel against the cut list.
2. Assemble with glue and pre-drilled, countersunk screws: back to bottom, then the sides, then the two port walls (they form the L-shaped slot along the left side and the back), then the baffle.
3. Seal every inside joint with silicone. An air leak in a ported box shifts the tuning and can whistle.
4. Leave the top unglued for now — screw it on dry, so you can fine-tune the port in step 7.
5. Finish: terminal cup in the back panel (sealed), carpet or paint.

A real box usually tunes 1–3 Hz lower than the design. You can check it with a phone and a flashlight.

1. Mount the sub, connect your phone to the head unit (AUX or Bluetooth), keep the volume low.
2. Open a bass tone generator (I used this one: https://subboxcalculator.com/tone-generator/) and start at about 45 Hz.
3. Go down 1–2 Hz at a time. Watch the cone and feel the air at the port.
4. At the tuning frequency the cone almost stops and the port blows hardest. That's your real tuning.
5. If it's lower than planned, take the top off and shorten the port wall along the back (panel 7) a little at a time — a shorter port tunes higher. When it's right, glue the top and seal the last joints.

While you're at it, sweep 20–80 Hz at moderate volume and listen for rattles (license plate, trunk lid, rear deck). Set the amp's subsonic filter to about 0.7–0.8 of the tuning — around 24–26 Hz for a 33 Hz box. Below the tuning the cone is barely loaded and can bottom out.

1. The SAv3-12 D2 has two 2 Ω coils: in series it's a 4 Ω load, in parallel 1 Ω. Pick what your amp is stable at.
2. Mount the sub with all the screws and even pressure so the gasket seals.
3. Strap the box down — a 45 lb box becomes a missile in a hard stop.
4. Set the gain with a test tone, not by ear, and start with the bass boost off.

If you build a box from this, I'd like to see it — post a photo in the comments. And if your sub isn't in the database, tell me the model and I'll add it.