Calculators
Box Calculator
Sealed, ported and passive radiator boxes for DIY speakers
Your speaker
These come from the Thiele/Small (T/S) table on your speaker's spec sheet. The example presets are typical values for that size, so swap in your real numbers when you have them.
What speaker are you using?
Pick an example to try the calculator, or choose “My own speaker” and type in the three numbers from your spec sheet. Look for a table called Thiele/Small or T/S parameters.
Pick a box type
Sealed
An airtight box. Tight, clean bass that rolls off gently. Most forgiving to build.
- Size
- Smallest
- Bass depth
- Good
- Difficulty
- Easy
Ported
A tuned tube lets the back wave help out. More output and deeper bass from the same speaker.
- Size
- Bigger
- Bass depth
- Deepest
- Difficulty
- Medium
Passive radiator
A weighted cone with no magnet does the port's job. Great when a port won't fit inside a small case.
- Size
- Compact
- Bass depth
- Deep
- Difficulty
- Medium
How do you want it to sound?
Each option is a real alignment, just named for what you'll hear. Bigger boxes go deeper.
A few build details
The calculator sizes the port for your amp power so it doesn't whistle. Two smaller ports are easier to fit than one big one.
Most radiators list size and weight. Use two radiators per speaker, or one with at least twice the cone area, so they don't run out of travel.
Your box
Predicted bass response
Build it
Predicted bass response
Predicted bass response
Predicted bass response
Measure a box
Find how much air a case or box actually holds. Works for Apache and Pelican style cases too: measure the inside of the speaker chamber.
Size a box
Start from the volume a calculator gave you and get inside dimensions to cut.
Before you start
Get the spec sheet for your speaker. Every decent speaker lists a table of Thiele/Small parameters, usually shortened to T/S. You only need three numbers to design a box:
- Fs
- The speaker's resonant frequency in free air, in Hz. Lower Fs usually means it can play deeper.
- Qts
- A single number for how well the cone is damped. It's the main thing that decides which box type suits the speaker.
- Vas
- How stiff the suspension is, given as a volume of air. Listed in liters or cubic feet.
For Pro mode checks you'll also want Sd (cone area), Xmax (how far the cone can travel) and sensitivity (dB at 1 watt, 1 meter). If a spec sheet doesn't list Fs, Qts and Vas, the speaker wasn't made for a box and results will be a guess.
Which box type?
| Sealed | Ported | Passive radiator | |
|---|---|---|---|
| Sound | Tight, accurate, gentle roll-off | Louder and deeper, steeper roll-off | Like ported, very clean with no port noise |
| Box size | Smallest | Largest | Between the two |
| Best Qts | 0.5 and up | 0.2 to 0.5 | 0.2 to 0.5 |
| Watch out for | Air leaks | Long ports that won't fit, port whistle | Getting the weight right, radiator travel |
| Good for | Car audio, small builds, high Qts speakers | Max output, bigger boomboxes | Small boomboxes and cases where a port won't fit |
Quick rule: Qts above about 0.55, go sealed. Qts below about 0.45, go ported or passive radiator. In between, either works and it comes down to space and taste. If your spec sheet lists Qes, Pro mode also checks the Efficiency Bandwidth Product (Fs ÷ Qes). Under 50 leans sealed, over 90 leans ported.
Using the calculator
- Start in Guided build if this is your first box. It walks you through every choice.
- Enter Fs, Qts and Vas, and how many speakers share the same air space. Two speakers in one chamber need twice the volume.
- Pick the box type and the sound you want. The calculator picks the volume and tuning for you.
- Read the build volume. That's the inside space to build, with room already added for the speakers, port and anything else inside.
- Go to Box volume and turn that into inside dimensions, or check that the case you already have is big enough.
- Switch to Pro mode to set your own volume or tuning, check cone travel and port air speed, and add a high-pass filter like you would in your DSP.
Net volume vs. build volume
The calculator's main number is net volume: the air the speaker actually works against. Everything sitting inside the box steals some of it. The speaker's magnet and basket, the port tube, braces, an amp board, a battery pack. Add all of that back and you get the build volume, which is the inside space you actually cut or measure for.
If your amp and battery live in their own sealed compartment, leave them out. If they sit in the same space as the speakers, measure them and add them in. A battery that's 15 × 7 × 6 cm takes up 0.63 L.
Getting the build right
- Seal it. Sealed and passive radiator boxes depend on being airtight. Run silicone or hot glue on inside seams, gasket the speakers, and seal around every wire hole.
- Port placement. Keep the inside end of a port at least one port diameter away from any wall. Flared or rounded ends cut down on chuffing. 3D-printed flared ports work well.
- Long ports. If the port is longer than the box is deep, bend it into an L, run a slot along a wall, or switch to passive radiators.
- Passive radiator weight. Add weight evenly around the center. More weight means lower tuning. Adjust a few grams at a time.
- Stiff walls. Thin plastic walls flex and eat bass. Brace large flat panels, especially on cases.
- Poly-fill. Loose fill in a sealed box makes it act a little bigger and calms ringing. Don't block ports or passive radiators with it.
- High-pass filter. Ported and passive radiator boxes stop controlling the cone below tuning. Set a high-pass in your DSP a little below Fb to protect the speakers.
- Test it. Play a slow sine sweep from about 20 Hz upward. Whistling means a leak. At the tuning frequency the woofer should barely move while the port or radiators do the work.
Words you'll see
- Qtc
- The Q of the speaker once it's in a sealed box. 0.707 is the flattest response. Lower is tighter, higher is boomier.
- Fb
- Tuning frequency of a ported or passive radiator box. Around here the port or radiator does most of the work.
- F3
- Where the bass is 3 dB down from the rest. The lower, the deeper the box plays.
- Fc
- Resonant frequency of the speaker in a sealed box.
- Sd
- Effective cone area.
- Xmax
- How far the cone can move one way before it distorts.
- Port speed
- How fast air moves through the port. Under 17 m/s is quiet. Over 25 m/s you'll hear chuffing.
- Notch
- A passive radiator box has a sharp drop where the weighted radiator resonates on its own. Below it, output falls off fast.
- QL
- How leaky and lossy a ported box is. 7 is a normal box.