Box Calculator

Speaker Box Calculator — Built Not Bought BoomBoxes
Free Tool

Speaker Box Calculator

Calculate dimensions and tuning for ported, sealed, and passive radiator DIY speaker enclosures.

No speaker specs needed. Just enter your box dimensions and port size — get an instant tuning estimate. Great for checking a build before you cut wood.
Box Type
Box Dimensions
e.g. 0.125 = 1/8", 0.25 = 1/4"
Port
Quick Calc Results
Volume:
Imperial
Tuning Frequency
—Hz
Net Internal Volume
—L
Gross Box Volume
—L
Box Type—
Outer Dimensions—
Wall Thickness—
Port Area—
Port Length—
Driver Displacement (est.)—

Calculations use standard Thiele-Small formulas. Ported: port length formula with end correction k=0.85. Sealed: system Q from closed-box alignment theory; fill factor approximates acoustic Vas increase from polyfill damping. Results are theoretical — always prototype and measure with a calibrated mic. For plastic enclosures, add internal bracing and damping material.

How to Use the Passive Radiator Calculator

A plain-English walkthrough for DIY builders — no engineering degree required.

What is a Passive Radiator?

A passive radiator (PR) is basically a speaker cone with no motor — no voice coil, no magnet, just a surround, spider, and cone. It sits in your box right next to your active driver and acts like a tuned port, but without a tube. When your driver pushes air, the PR moves sympathetically and reinforces bass output at a specific frequency.

Instead of cutting the box length to change tuning (like a port), you tune a PR by adding or removing mass — usually small weights, washers, or acoustic putty stuck to the cone. More mass = lower tuning frequency. Less mass = higher tuning frequency. That's the whole game.

✓ PR Advantages
  • No port chuffing or wind noise at high volume
  • Tunable after the box is already built
  • Works great in small or odd-shaped enclosures
  • Clean look — no port hole cutting required
  • Can go very low in compact boxes
✗ PR Trade-offs
  • Requires a separate PR part (adds cost)
  • Less efficient than a well-tuned ported box
  • Needs accurate PR specs to calculate well
  • Box must be completely airtight — any leak kills bass
  • Less common, fewer build references online
Step-by-Step: Using the Calculator
1
Enter Your Driver's Thiele-Small Parameters

Pull these from your speaker's spec sheet or the manufacturer's website. You need Fs (free-air resonance in Hz), Vas (equivalent air volume in liters), and Qts (total Q factor).

Ideal Qts for PR: Drivers with Qts in the 0.25–0.45 range work best. Higher Qts drivers tend to be boomy and poorly controlled in a PR box.
2
Enter Your Passive Radiator Specs

You'll need the PR's diameter, Cms (compliance in mm/N), Mms (moving mass in grams — this is the base mass before you add weight), and Xmax. Check the PR datasheet.

Don't have a datasheet? Use these starting estimates for a generic PR the same size as your driver:
8" PR
Cms ~2.0 mm/N
Mms ~50 g
10" PR
Cms ~1.5 mm/N
Mms ~80 g
12" PR
Cms ~1.0 mm/N
Mms ~120 g
3
Set Your Target Tuning Frequency and Hit the Button

Enter your desired tuning frequency (Fb) in the Target Tuning field, then click Calculate Required PR Mass for Target Fb. The calculator will automatically fill in the Mms field with the total mass your PR needs to hit that frequency.

Typical tuning targets: 28–35 Hz for deep bass in a BoomBox build. Going too low (under 25 Hz) in a small box will make the bass sound loose and weak. A good starting point is tuning 5–8 Hz below your driver's Fs.
4
Read Your Results and Add Mass to the PR

The Required Added Mass field tells you how many grams to add to your PR on top of its stock weight to hit your target Fb. The Actual Fb shows what the system will be tuned to based on the current Mms value.

When adding mass: distribute it evenly around the cone (not all in one spot), use non-magnetic material like acoustic putty or lead-free fishing weights, and make sure it's secured so it can't rattle or fly off.

Understanding the Results
Actual Fb — Tuning Frequency

The frequency at which the PR system resonates. This is where you get your bass boost. Below this point, output drops off quickly — similar to a ported box.

Required Added Mass

Extra grams to attach to the PR cone. The calculator shows what you need beyond the PR's stock moving mass. Start at 80% of this number and fine-tune by ear.

Fpr — PR Resonant Frequency

The PR's own free-air resonance with its current mass. This is separate from the system Fb — Fpr is just a component property, not your tuning frequency.

PR to Driver Area Ratio

The PR's cone area vs. the driver's total cone area. Aim for 1.0:1 or higher. More PR area means cleaner excursion and less distortion at high volume. Two smaller PRs often beat one big one here.

PR to Driver Area Ratio Guide
Ratio
Sound Character
Verdict
Below 0.8:1
PR overworks, distortion at high volume
Avoid if possible
0.8–1.0:1
Acceptable, modest output ceiling
Workable
1.0–1.5:1
Balanced performance, clean at volume
Good
1.5–2.0:1
PR handles excursion easily, maximum headroom
Ideal
Troubleshooting Common PR Problems
PR bottoming out / slapping
  • PR Xmax is too small for the driver
  • Add a second PR to share the excursion load
  • Raise tuning frequency slightly (remove a little mass)
  • Reduce power or add a subsonic filter below Fb
Loose, boomy, or one-note bass
  • Check for air leaks — every joint, every screw hole
  • Box volume may be too large for your driver
  • Tuning may be too low — remove some mass from the PR
  • Add light polyfill inside the box for damping
Not enough bass output
  • PR cone area too small relative to driver — use a larger PR or add a second
  • Box volume may be too small — increases Fb, lifts the tuning point
  • Tuning too high — add more mass to lower Fb
Rattling or buzzing from the PR
  • Added mass is not secured — reglue or re-putty
  • PR mounting screws are loose — check all fasteners
  • PR surround or spider is damaged — inspect visually
⚡ Builder Tips from Built Not Bought
🔩 Start with 80% of the calculated mass. Add the remaining 20% by ear — every box is slightly different and real-world tuning beats a perfect calc.
⚖️ Use acoustic putty for mass. It's easy to shape, sticks to the cone without adhesive, stays balanced, and is completely reversible. Fishing sinkers work too.
🔇 Seal like your life depends on it. Use silicone on every seam, every screw hole, every grommet. A PR system with even a small air leak sounds dead below tuning.
📐 Size up on the PR when you can. A PR one size larger than your driver is better than same-size. More cone area = less excursion = less distortion = more output.
🎚️ Add a subsonic filter. Set it around your Fb. Below tuning, the driver is unloaded and can bottom out fast. A subsonic filter at 20–25 Hz protects it.
🧪 Tune by listening, not just calculating. Play bass-heavy music at moderate volume. If the PR slaps or bottoms, add mass or lower power. If bass sounds thin, try removing mass.