Formula Used
Internal width = outer width − 2 × material thickness.
Gross volume = internal width × internal height × internal depth.
Net volume = gross volume − driver displacement − bracing displacement − terminal displacement − port displacement.
Round port area = π × radius² × number of ports.
Slot port area = slot width × slot height × number of ports.
Port length estimate = ((14630000 × equivalent radius²) ÷ (tuning² × box cubic inches)) − (1.463 × equivalent radius).
Sealed Qtc estimate = Qts × √((Vas ÷ net volume) + 1).
Sealed Fc estimate = Fs × √((Vas ÷ net volume) + 1).
How to Use This Calculator
Choose inches or centimeters first. Enter the outside box dimensions and material thickness. Add the driver, bracing, and terminal displacements. Select sealed or ported design. For sealed boxes, add Fs, Qts, Vas, and target Qtc when available. For ported boxes, choose round or slot port details and enter the target tuning. Leave known port length blank when you want the calculator to estimate it. Press calculate. Review the result above the form. Use the CSV or PDF buttons to save your design summary.
Subwoofer Box Design Guide
A subwoofer box controls the way bass behaves. The driver is only one part of the system. Air inside the enclosure acts like a spring. Panel size, port area, and displacement change that spring. A small error can raise tuning or reduce output.
Why Net Volume Matters
Gross volume is the raw inside space. Net volume is the space left after losses. The driver basket, bracing, terminal cup, and port all remove airspace. This calculator separates those values before showing the final volume. That makes planning easier before wood is cut. For sealed boxes, net volume affects damping and response shape. A smaller sealed box usually gives tighter control. A larger sealed box can extend low bass, but may reduce power handling. For ported boxes, net volume works with the vent to set tuning. The tuning point shapes output near the low end.
Port Planning
Port area should be large enough for air movement. Too little area can create noise at high output. Round ports are simple to build. Slot ports can fit wide, shallow boxes. The port length estimate uses the selected net volume and target tuning. It also includes an end correction for practical vent behavior. Very long ports may need folds inside the cabinet. Always keep clearance around the inside port opening.
Building Notes
Use the cut list as a planning guide. It assumes a simple rectangular box with butt joints. Real builds may need kerf allowance, carpet wrap, double baffles, or recessed mounting. Measure the actual driver and port parts before final cutting. Bracing should be strong, but it must be counted as displacement. Seal every joint with glue and caulk. Air leaks can reduce bass and create rattles. Use the same measurement system through the whole form. Review the result table before downloading a file. Good design starts with careful numbers. Good sound comes from careful construction.
Best Use
This tool is useful for car audio, home theater, and workshop planning. It gives practical estimates, not a final acoustic guarantee. Manufacturer recommendations should still guide the design. Room gain, cabin gain, amplifier power, and driver limits also matter. Use the calculator to compare options. Then test with moderate power first.
FAQs
What is net volume?
Net volume is usable airspace after subtracting the driver, bracing, terminal cup, and port volume. It is usually more important than gross volume because it controls the actual acoustic behavior of the enclosure.
Should I choose sealed or ported?
Choose sealed for simpler builds and controlled response. Choose ported when you need more output near the tuning frequency. The best option depends on the driver, vehicle, room, power, and listening goal.
Why is port length sometimes very long?
Low tuning, small net volume, and large port area can create a long vent. A folded slot port or a larger box may be needed when the straight port cannot fit.
Can I ignore driver displacement?
No. The motor, basket, and cone occupy space inside the cabinet. Ignoring displacement makes the final box smaller than planned and can change tuning or sealed response.
What does Qtc mean?
Qtc describes sealed system damping. Near 0.707 is a common balanced target. Lower values can sound smoother. Higher values can sound punchy, but may create a peak.
Does polyfill increase real volume?
Polyfill does not change the physical size. It can make a sealed box behave like it is slightly larger. This calculator applies it only as an effective sealed volume estimate.
Is the cut list final?
The cut list is a basic rectangular planning guide. Adjust it for your joinery, saw blade width, double baffles, rabbets, carpet wrap, and actual panel layout.
Can this replace manufacturer box specs?
No. Use this tool for planning and comparison. Manufacturer recommendations, driver limits, and real measurements should guide the final enclosure before cutting material.